[548] Murch. Sil. Syst. p. 588; see also a detailed description of the Cephalaspis, Holoptychius, and other Devonian Ichthyolites, by M. Agassiz, ibid. p. 589-601.
Fossil Sauroid Fishes. (Poiss. Foss. tom. ii.)—The family of Ganoid, fishes termed Sauroid, or lizard-like, by M. Agassiz, are so named in consequence of certain peculiarities in their organization which are found in no other animals of this class, but exist in reptiles. There are but two living genera; namely, the Lepidosteus, of which there are many species that inhabit the rivers of America; and the Polypterus, that contains two species, one inhabiting the Nile, and the other the rivers of Senegal. In these fishes the bones of the skull are closely connected by sutures; the teeth are large, conical, and longitudinally striated, as in the crocodile, plesiosaurus, &c.; the spinous processes are united to the bodies of the vertebræ by suture, as in most reptiles; and the ribs are articulated to the extremities of the transverse processes; the skeleton is osseous. Even in the soft parts many analogies to reptilian structure are seen; thus the Lepidosteus has a glottis, as in the Siren; and a cellular air-bladder, with a tracheal vessel, resembling the lungs of an Ophidian (serpent). These fishes are the only living representatives of those voracious tribes of the most ancient marine fauna, whose remains abound in the Secondary formations. Their relics have often been mistaken for those of reptiles; particularly the teeth, which from their large size, conical figure, enamelled and striated surface, and internal cavity, were generally supposed to belong to crocodiles. The scales are flat, rhomboidal, and parallel to the body. The recent Lepidosteus osseus, of North America, affords a good illustration of the fossil genera; a reduced figure of this fish, from Poiss. Foss., will be found in Bd. pl. xxviia.; and teeth of some fossil Sauroids are represented Bd. pl. xxvii.
The teeth of the Sauroids consist of two kinds; namely, large pointed striated cones, and numerous small brush-teeth. The intricate structure of the conical teeth of the Stony-gar, or Lepidosteus, is very remarkable, and presents some analogy to that observable in the dental organs of the Labyrinthodon, an extinct genus of reptiles, of which we shall treat in a subsequent chapter. The tooth consists of a large conical pulp-cavity, surrounded by a mass of dentine, which is plicated longitudinally, its folds giving to the pulp-cavity an appearance of being divided into parallel longitudinal branches; resembling, in this respect, the base of the tooth of Ichthyosaurus, as shown in a transverse section, [Pl. VI. fig. 9]. If we imagine these folds to be multiplied, and to have more inflections, and the pulp-cavity to be reduced in its proportions, we shall have the elegant organization of the teeth of the Labyrinthodonts (see [Pl. VI. fig. 3b]). The dentine is composed of very minute calcigerous tubes, which pass off at right angles from the pulp-cavity to the periphery; and it is covered by a layer of cement, or coarser dentine, which is encased in a coat of enamel, forming the external investment of the tooth.[549] The long conical teeth are implanted in alveoli or sockets, to the walls of which they are anchylosed at their base.
[549] These remarks are based on M. Agassiz’s description and my own observations. An interesting paper, "On the Microscopic Structure of the Teeth of the Lepidostei, and their analogy with those of the Labyrinthodonts, with a plate," by Dr. Jeffries Wyman, will be found in Amer. Journ. of Science, October, 1843, p. 359.
Lepidostei.—This family, having the above described recent Lepidosteus for its type, comprises several genera, and is represented in all the formations from the Tertiary to the Coal-measures inclusive. The Lepidosteus itself has left its remains in the Eocene tertiary of England. The Lepidotus (before described, [p. 604],) ranges from the Chalk to the Lias; the Pholidophorus and Dapedius ([p. 603]) abound in the Lias; the latter being found also in the Wealden, and the former in the Oolite and Purbeck; and the Palæoniscus (see [p. 601]) is a well-known Permian and Carboniferous form.
Sauroidei.—The remains of the Sauroids proper occur in the Chalk, Purbeck, Oolite, Lias, Permian, and Carboniferous rocks. The great strength and size of some of these teeth prove that the seas of those remote periods were inhabited by voracious fishes of enormous magnitude. See Petrif. pp. 432, et seq.
The teeth and jaw of a gigantic sauroid (Megalichthys), from the Carboniferous strata at Burdie-house, are figured Bd. pl. xxvii.; the sections of the teeth shown in figs. 13, 14, of that plate, illustrate the size of the pulp-cavity and the thickness of the layer of dentine. These remains were associated with the plants and crustaceans previously described; an assemblage of fossils indicating a lacustrine or estuary formation. Similar teeth have been found in the cannel-coal of Fifeshire.
In the Upper Lias of Ilminster, Somerset, the nodular calcareous rock is rich with the remains of the Pachycormus, many fine specimens of which Mr. C. Moore, of that town, has successfully worked out from their stony encasement, exhibiting the gaping, contorted fish, as it died in the convulsive throes of suffocation in the muddy sea. The little sauroid Leptolepis, of the Lias and the Purbeck, is also abundant in the above mentioned locality.