PLACOID ORDER-In the placoid fishes the skeleton is cartilaginous, the gills are fixed, the skin is not covered with scales as in the other three orders, but either studded with bony tubercles or scutcheons of enamelled bone, or protected by very small bristly plates, constituting a tesselated integument, called shagreen. In consequence of the perishable nature of the skeleton, the teeth, spines, or fin-rays, tubercles and scutcheons, vertebræ, and in some rare instances the dermal shagreen, are the only parts preserved in a fossil state. The Placoids are the most ancient animals of the vertebrated classes hitherto discovered, for rays of a species of shark (Onchus) have been found in the Lower Silurian deposits; and they have continued through the entire series of formations, and abound in the present seas. But though the fishes of this order are the most universally distributed in time as well as in space, the relative numerical predominance of the several families varied greatly in different periods.

In Table-Cases 2, 4, 6, 8, 10, the collection of teeth, vertebræ, fin-rays, or Ichthyodorulites, &c. are arranged under the respective families to which they belong.

CHIMEROIDS.-Table-Case 4.-A good collection of the maxillary

LIGN. 95.-MANDIBLE OF A CHIMEROID FISH;
EDAPHODON MANTELLI, FROM THE CHALK,
LEWES. (nat. size.)

organs, and the hard indivisible plates of dentine, composing the dental instruments of this curious tribe of the Shark family, belonging to four subgenera, is deposited in Table-Case 4. They are referable to Ischyodus, Edaphodon, Psittacodon, and Ceratodus. Among them are beautiful examples from the Eocene clay of Bracklesham, in Sussex, from Mr. Dixon's

collection, and figured and described in his work by Sir Philip Egerton. There is one pair of mandibles in a block of chalk, remarkable for their prolongated and curved form, which led M. Agassiz to name the genus Psittacodon; the Edaphodon, of Sir P. Egerton.1

SQUALIDE, or SHARKS.-The fossil teeth of this universally distributed family of voracious fishes, are abundant in almost every secondary and tertiary deposit. Want of space compels me to refer to "The Medals of Creation," for a popular account of these fossil relics.2 The collection is very rich in the usual types of the genera and subgenera. The sharp triangular teeth, with or without lateral denticles, and the cutting edges, either smooth or serrated, occur in profusion in many tertiary strata, and species of the same genera abound in the chalk. (Lign. 96.) HYBODONS.-A family allied to the Sharks, but with conical and uncompressed teeth. These fishes had two dorsal fins, with anterior

See "Geological Journal," May 1847.

2 Chap XV. p. 611.

spines; the ichthyodorulites of this family are abundant in the Wealden, Oolite, &c.

1. Otodus.

5. Notidanus.

3. Corax, 4. Oxyrhina.
7. Acrodus.

2. Carcharias.

6. Lamna.

8. Pycnodon; tooth of a ganoid fish; upper surface.
9. Lateral view. 10. Ptychodus.

CESTRACIONS.-Of this family a few teeth have been found in the Kentish Chalk, that belong to fishes of the existing genus Cestracion, or Port Jackson Shark. A genus of this family, named Ptychodus (rugous-tooth,) must have swarmed in the cretaceous ocean, for its large, quadrate, channelled molars, are found in every quarry of the White

LIGN. 97. TEETH OF HYBODUS.

Chalk (Lign. 96, 10); there are groups of from ten to fifty on blocks of chalk in the Table-cases.

Of the PRISTIDES, or Saw-fishes, there are a few teeth, and portions of the beak, of two species, from the Eocene of Bracklesham.

FOSSIL RAYS.-Instead of pointed teeth, the masticatory organs of the fishes of this family are wide, flat, dental plates, composed of distinct

G G

pieces in close juxtaposition and united by fine sutures. There are fifteen fossil species of the Myliobatis (Eagle-ray), in some of which the dental plates are of great size, as shown by specimens in the Table-case under review. There are likewise some fine examples of the allied genus, Etobatis, from Bracklesham.

ICHTHYODORULITES, or Fossil Dorsal-fin-spines.—Table-cases, 2, 4, 6.— The fossils thus named are the rays or spines of the dorsal fins of cartilaginous fishes; of these, there is an extensive series in the collection, as specified in the List of Genera. I would direct attention to some beautiful specimens in the Table-case 2, and especially to the large spine of Ptychodus spectabilis in Chalk, formerly in my collection, and one of the most interesting fossils of this kind hitherto discovered.

GEOLOGICAL DISTRIBUTION OF FOSSIL FISHES.-From the incidental notices of the geological habitats of the Ichthyolites described in the preceding pages, the intelligent reader cannot fail to perceive that the most recent deposits abound in forms allied to the existing genera and species, while the most ancient teem with unknown families, which are either utterly extinct, or of excessive rarity at the present time. M. Agassiz affirms that in a general sense we may conclude that the Ichthyolites of the Tertiary deposits approach in their character to the living genera, but all the species are extinct. That the newer Tertiary, as the Crag, contain genera common to tropical seas, as the large sharks (Carcharias), and eagle-rays (Myliobates), &c. In the Eocene, as the London and Paris basins, Monte Bolca, &c. one-third of the Ichthyolites belong to extinct genera. Of the Chalk fishes, two-thirds are of extinct genera, but related to those of the Tertiary formations. From the Oolite to the Lias, including the Wealden, the fishes constitute a natural group, but few species of which occur in the Chalk, and all the ganoid fishes are homocercal; below the Lias, a prodigious number of unknown genera and species appear, and these are almost all heterocercal.

Thus of the eight thousand living fishes known to naturalists, threefourths belong to the Cycloid and Ctenoid orders, and of these no species occur below the Chalk; the other fourth is referable to the Placoids and Ganoids, of which there are comparatively but few existing species. Yet fishes of these two orders were almost the sole representatives of the Class Pisces, during the ancient secondary formations; for below the Lias, the predominant recent orders are altogether absent. Beneath the Coal true carnivorous fishes, with trenchant teeth, are almost unknown; omnivorous species, with either brush or obtusely conical teeth, and great sauroid fishes, are the prevailing representatives of the class. In fine, the Ichthyolites of the different formations constitute two grand groups, which have their boundary line at the base of the Cretaceous deposits. The first and most ancient comprises the Ganoids and Placoids; the second is more intimately related to existing types, and comprehends forms more diversified; these are principally Ctenoids and Cycloids, with a small number of the two preceding orders, which insensibly disappear, and their few living analogues are very distinct from the ancient species.

Now, although deductions of this nature may require to be modified with the progress of knowledge, yet the generalizations thus obtained are founded on so great an accumulation of facts and observations, as to

render it improbable that they will be materially invalidated by future discoveries; for they accord remarkably with the results derived from the investigation of the fossil remains of the other Classes of animals. The most modern deposits contain the remains of animals allied to species of existing genera; the most ancient, of forms altogether extinct, or of excessive rarity in the recent faunas. The discovery of existing species, or genera, in the most ancient strata, would modify, but not destroy, the inferences deduced from the facts hitherto obtained; and every geologist is prepared to find that such may be the case.

Thus, of the sharks, with serrated triangular teeth, whose relics are so common in the Tertiary formations, and were formerly unknown in the ancient Secondary, one representative has been found in the Carboniferous system; but, if teeth of this character should hereafter be discovered in other Secondary deposits, the great preponderance of this type in the Tertiary strata, and in the existing seas, would not be the less remarkable.

LIST OF THE GENERA OF FOSSIL FISHES, OF WHICH THERE ARE EXAMPLES OF ONE OR MORE SPECIES IN ROOM V. OF THE BRITISH MUSEUM.

WALL-CASE A.-(1, to 5.)

Coccosteus. (C. decipiens, C. cuspidatus, from the Devonian of Cromarty; C. oblongus, from Lethenbar.)

Cephalaspis. (C. Lyellii, from the Devonian formation, Forfarshire.) Gyrolepis. (G. Rankinii.)

Pholidophorus. (P. taxis.)

Amblypterus. (A. macropterus.)

Pamphractus. (P. hydrophilus.)

Diplacanthus. (D. striatus.)

Cheiracanthus. (C. microlepidotus.)

Pterichthys. (P. oblongus.)

Platysomus.

Cheirolepis. (C. Cummingiæ, from Devonian strata, Cromarty.)

Acanthodes. (A. Brownii.)

Osteolepis. (O. intermedius, Devonian, Cromarty.)

Dapedius. (D. politus, D. Colei, lias of Dorsetshire.)

Tetragonolepis. (T. speciosus, from the lias, Lyme Regis.)

Semionotus. (S. Bergeri, a group of several, in lias, Coburg.)

Palæoniscus. (Several from the Zechstein of Thuringia.)

Lepidotus. (L. Fittoni, L. Mantelli, Wealden of Tilgate Forest.)

WALL-CASE B.-(6, to 14.)

Dipterus. (D. macrolepidotus.)

Leptolepis. (Several species from the Oolite, Solenhofen.)
Sauropsis. (S. latus.)

1 Carcharopsis prototypus.

Megalichthys. (M. Hibbertii, carboniferous, Burdie House, near Edin

burgh.)

Rhizodus. (R. Hibbertii.)

Pygopterus. (P. Humboldtii, copper slate of Mansfeld.)

Megalurus. (M. lepidotus.)

Eugnathus. (E. chirotes, lias of Wirtemberg.)

Thrissops. (T. formosus.)

Ptycholepis. (P. Bollensis, lias of Wirtemberg.)

Pachycormus. (P. gracilis.)

Caturus. (C. dubius, C. macrodus.)

Saurichthys. (S. apicalis.)

Acanthopleurus. (A. serratus.)

Aspidorhyncus. (A. acutirostris, very fine, from Monte Bolca.)

Platygnathus. (P. Jamesoni.)

Belonostomus. (B. cinctus, from the chalk near Lewes.)

Acrolepis. (A. asper, from the Keuper schist.)

Diplopterus. (No species labelled.)

Holoptychius. (H. Flemingii, H. nobilissimus, from Devonian sand

stone, Clashbinnie, near Perth.)

Glyptolepis. (G. leptopterus.)

Asterolepis. (A. Asmusii, Devonian of Riga.)

Bothriolepis. (B. priscus.)

Rhinellus. (R. furcatus.)

Macropoma. (M. Mantelli, Chalk formation, Lewes, Sussex.) Lophiostomus Dixoni. A new ganoid from the South Downs, (ante, p. 433.) Phyllolepis. (P. tenuissimus.)

WALL-CASE C.-(15, to 23.)

Dercetis. (D. elongatus, from the chalk at Lewes.)

Blochius. (B. longirostris, Monte Bolca; a very fine example.)
Acanthopleurus. (A. serratus, chalk formation, slate of Glaris.)
Pimelodus. (From the Sewalik Hills.)

Gyrodus. (S. angustus.)

Pycnodus. (P. gigas, from the Upper Oolite; P. Mantelli, Wealden.) Platysomus. (P. gibbosus.)

Placodus. (P. gigas, from the Muschelkalk of Franconia.)

Acrotemnus. (A. faba, Sussex chalk.)

Silurus. (S. Egertoni, eocene clay, Bracklesham, Sussex.)

Sphærodus. (S. gigas, from the Oolite.)

Phyllodus.

Microdon. (M. nuchatis, Lewes chalk; M. hexagonus, Monte Bolca.) Perca. (P. lepidota, from Eningen; very fine.)

Cyclopoma. (C. gigas, Monte Bolca.)

Pelates. (P. quindecimalis, Monte Boica.)

Serranus. (S. occipitalis.)

Beryx. (B. Lewesiensis, Mantell, B. ornatus, Agassiz, Lewes chalk, discovered in 1820.)

(B. radians, B. microcephalus, and B. superbus, from the Lewes chalk.)

Smerdis. (S. minutus, Eocene, Aix, Provence.)

Rhacolepis. (R. Brama, R. latus, from the chalk formation of Brazil.)