In the lithographic stone of Solenhofen, and in the Purbeck strata, Oxford Clay, and Lias of England, occur specimens of a sauroid fish remarkable for the extreme shortness of the lower jaw, and the prolongation of the upper into a beak; it is named Aspidorhynchus by Agassiz. The figure of this fossil fish is contrasted on pl. xxviia. Bd. with that of its recent ally, the Lepidosteus.
In the Shanklin sand and in the Galt of Kent and Sussex, large, conical, striated teeth belonging to sauroid fishes are occasionally found. They invariably occur detached, and no portion of the jaws has been observed. In the Chalk of Sussex several fine sauroid fishes have been discovered; such as the Lophiostomus,[550] Belonostomus,[551] and Caturus; the last two of which are found in the Oolite and Lias also. These have been described and figured in the late Mr. Dixon’s "Geology and Fossils of Sussex."
[550] Geol. Surv. Dec. 6. pl. x. xi.
[551] Petrif. p. 431.
The Sauroidei-dipterini[552] are found almost exclusively in the Devonian formation. Osteolepis, Diplopterus, and Dipterus are characteristic members of this family. Interesting descriptions and instructive figures of the structure of these genera are given in Mr. Hugh Miller’s late work, "Foot-prints of the Creator, or the Asterolepis of Stromness," at pp. 50 to 61.
[552] The characters of the scales of this and the next family, as well as of the Acanthodei, are succinctly given in Miller’s Foot-prints of the Creator, pp. 30, et seq.
Cœlacanthi.—This is a numerous family of sauroid fishes, that have derived their name, hollow-spine, from the central cavity in the fin-rays, which, however, may have had originally cartilaginous cores. They are found from the Devonian to the Cretaceous series. In the former, one of the most remarkable is the genus Holoptychius, distinguished by the peculiar structure of the scales; the enamelled surface of which is covered by undulated furrows. The whole body is covered by thick enamelled scales of this kind. A splendid specimen, twenty-eight inches long, and twelve wide, is figured Murch. Sil. Syst. pl. ii.bis. Scales have been found exceeding three inches in length, by two and a half in width; which must have belonged to a fish of great magnitude. Ly. fig. 395.
In the Old Red Sandstone of Elgin, at a quarry at Scat-craig, some peculiar teeth occur, which possess a very remarkable structure, and have been referred to a genus of sauroid fishes, which, from the dendritical or arborescent disposition of the calcigerous tubes, Prof. Owen has named Dendrodus. These teeth are of a conical form, slightly curved, and solid throughout. On the external surface they are finely striated longitudinally, and have two opposite vertical ridges; the base is rough, and its margin rounded, as if for attachment to a shallow socket. The largest specimen is one and a half inch in length. In thin sections, viewed microscopically by transmitted light, there is a central pulp-cavity, of small size; the medullary canals pass into a few short ramifications, like the branches of a shrub, and these are distributed into irregular dilatations, simulating leaves, which resolve themselves into radiating bundles of calcigerous tubes; a portion of a transverse section[553] is shown [Pl. VI. fig. 8].
[553] Odontography, pl. lxii. B fig. 2; and Cycl. Anat. Art. Teeth.