Lamna elegans. [Lign. 193], fig. 6.—The fishes of the genus Lamna (to which the recent shark called the Porbeagle belongs) have teeth with smooth trenchant edges, and a small sharp denticle (little tooth) on each side the base, as in the fossil, [Lign. 193], fig. 6. The specimen, fig. 2, although devoid of denticles, probably belongs to the same genus, for reasons already explained. Several species abound in the Chalk; and they are associated with teeth, which are relatively wider and shorter, and have large compressed denticles; the latter are arranged in a separate genus, named Otodus (eared-tooth), by M. Agassiz. The specimen figured [Lign. 192], fig. 5, represents O. obliquus; another species, Otodus appendiculatus, is abundant in the Sussex Chalk. The large, wide, triangular, smooth teeth, with trenchant edges, and destitute of lateral denticles, so common in the Chalk, are related to Lamna, and are comprised in the genus Oxyrhina (Poiss. Foss. tom. v. tab. xxxiii.).
Notidanus microdon. [Lign. 193], fig. 3.—These teeth differ remarkably from those of the other genera of Sharks. The crown of each tooth is composed of a series of sharp angular enamelled points, the first of which is the largest, and is notched on its anterior edge; the base or root is osseous, flat, with a slight longitudinal depression below the border of enamel. These teeth are comparatively rare in the Chalk. One species has been found in the Oxford Clay; and several in the Tertiary strata. Specimens occur in Hordwell Cliff.
Corax pristodontus. [Lign. 193], fig. 1.—The teeth of the fossil Corax chiefly differ from those of the recent genus Galeus, to which the Tope, or Grey Shark, belongs, in being solid; they are of a triangular form, with a deep concavity or notch on the posterior margin, the base of which is prolonged and forms three or four angular points: the anterior edge of the tooth is finely serrated. The root of the tooth, as in Notidanus, is a broad osseous plate. There is much diversity of form in the Chalk specimens, which are all of a small size, as in [Lign. 193], fig. 1. In Sussex they are more common in the Chalk-marl than in the Chalk.
The only fossil teeth of the Shark family resembling those of the tertiary Carchariodonts, that have been discovered in the strata below the Chalk, are from the carboniferous deposits of Yorkshire and Armagh. These teeth are compressed, triangular, crenated on the edges, with large plaits or folds on the enamelled surface, towards the base of the crown. M. Agassiz refers them to a new genus, viz. Carcharopsis, with the specific name of Prototypus.
Fossil Vertebræ of Sharks.—As the cartilaginous nature of the skeleton in this family renders it unfavourable to preservation in the mineral kingdom, the durable parts already described, and those which are ossified, are almost the only relics found in a fossil state. The dermal integument is, however, sometimes preserved; and I had a beautiful example of shagreen, composed of irregular minute hexagonal scales, one of which is represented highly magnified, [Lign. 185], fig. 1.
In the Galeus and Carcharias the vertebræ are more ossified than in many other genera of cartilaginous fishes, and fossil vertebræ of these sharks often occur in the cretaceous and other strata. Groups of vertebræ of a large size occasionally occur in the Sussex Chalk; they are circular, biconcave, and very short; one specimen is four inches in diameter, and one inch long; their concavities are consequently shallow. These vertebra: are composed of two shallow conical disks, which are united by their summits, at the axis, and are connected and supported by numerous wedge-shaped plates, that radiate from the centre to the periphery (see Foss. South D. pl. xxxiii. fig. 10). My collection contained a connected series of forty small vertebræ from the Chalk near Lewes, which probably belonged to the same species of Shark as the dorsal spine named Spinax major (Poiss. Foss. tom. iii. pl. xla fig. 6).
Squaloraia. In the Lias of Lyme Regis, that inexhaustible storehouse of fossil treasures, a considerable portion of the skeleton of a very remarkable fish, partaking of the characters of the Sharks and Rays, was discovered by Miss Mary Aiming, and is now in the Museum of the Bristol Institution.[527] In this fish the jaws are prolonged into a beak, like the Pristis (Saw-fish). It has the head of a Shark, with a long beak; vertebræ of the Rays; with pectoral and ventral fins, almost equally developed; a tail armed with, a spine; and spinous bosses, as in the true Rays.
[527] It is figured and described by Dr. Riley, Geol. Trans. 2d ser. vol. v. pl. iv. See also Poiss. Foss. tom. iii. pl. xlii.
Fossil Pristis, or Saw-fish.—This well-known predatory fish, which is allied to the Rays and Sharks, and referred by M. Agassiz to the family of Raiidæ, has projecting from its snout an osseous, flat, horizontal plate, or beak, equal in length to one-third of the fish, and armed on each side by a row of elongated, compressed, pointed teeth, implanted in sockets; the front margin of these teeth is convex, the posterior concave; this defence is termed the saw, and constitutes a most powerful weapon. The Pristis has also numerous small obtuse teeth on the jaws. The remains of the beak, or saw, of an extinct species of Pristis have been discovered in the Bagshot Sand at Goldsworth Hill, Surrey,[528] and three other species have been found in the London clay of the Isle of Sheppey, and the Eocene beds at Bracklesham and Hordwell.[529]