The evidences of plagiostomous fishes afforded by fossil spines will be here pursued. In most of the existing cartila ginous fishes of this order the defensive spine which stands erect in front of the dorsal fin is smooth ; such is the case in the dog-fishes (Spinacidce) in which each dorsal fin is fronted with a spine. In the Port-Jackson sharks (Cestraciontidce) the spine in front of each dorsal is bony, and is armed along its hinder or concave border with bent spines. The fin is connected with this border, and its movements are regulated by the elevation or depression of the spine during the peculiar rotatory action of the body of the shark. This action of the spine in raising and depressing the fin, resembles, Dr. Buckland has remarked, that of the moveable or jointed mast, raising and lowering backwards the sail of a barge. But their more obvious use, in the small Plagiostomes possessing such spines, is as defensive weapons against the larger and stronger voracious fishes. The spine of the Onclms indicates its danger from some larger, and as yet unknown, predatory fish.
Certain bony fishes are similarly armed—e.g., sticklebacks (Gasterostei), sheat-fishes (Siluricke), trigger-fishes (Balistes), and some species of snipe-fishes (Fistularidce). In the latter family the Centriscus humerosus (fig. 23) shows a dorsal spine, denticulated behind, as in the Cestracionts ; but the base of the spine in bony fishes is peculiarly modified for articulation with another bone. In the Plagiostomes the base of the spine is hollow, becomes thin and smooth when the body of the spine is sculptured, and is in the recent fish implanted in the flesh.
The following genera of plagiostomous fishes have been founded on the fossil spines, or " ichthyodoruliths," which have been discovered in the " Devonian," or " Old Red Sandstone series." Onchus (represented by 0. semistriatus, 0. h.eterogyrus), Dimsracanthus, Haplacanthus, Narcodes, Naulas, Bysscicanthus, Cosmacanaus, Homacanthus (fig. 24), Ctenacanthus, Ptyacan thus, Climatius, Parez-us, Odontacanthus, and Pleuracanthus.
The genus Homacanthus is founded on small compressed spines, with fine recurved teeth on the back edge, and longitudinal striae on the sides. Specimens of Homacanthus arcuatus (fig. 24) have been found in Devonian formations near St. Petersburg.
The carboniferous series of formations includes the mountain limestone, millstone grit, and the coal measures (see fig. 1). In this series the genus Onchus is still represented by the 0. sul calm, 0. rectus, and 0. subulatus ; and the genus Homacanthus, by H. macrodus and H. microdus, from the carboniferous limestone of Armagh. Ptyacanthus, Ctenacanthus, and Pleuracanthus are also forms common to the Devonian and carboniferous periods. The spine of the latter genus is denti culated along both margins, a structure which is presented, in existing Plagiostomes, only by species of the ray family ; Plewracanthus, therefore, as Agassiz concludes, may offer the earliest example of the flat form of cartilaginous fish which is represented by the sting-rays (Trigon, Myliobates) in the present seas. The ichthyodorulite (ichthys, a fish ; dora, a spear ; lithos, a stone) here selected to illustrate this fossil, is a portion of the spine of the Pleuracanthus levissimus (fig. 25), from the
carboniferous beds near Dudley. The other plagiostomous genera based upon fossil spines from the coal formations are,—Ora,canthus, Gyra canthus, Nemacanthus, Cosmacanthus, Leptacan thus, Homacanthus, Trysti,chius, Asteropterychius, Physonemus, Sphenacanthus, Platyacanthus, Dip riacanthus, Erismacanthus, Orthacanthus, Cladacanthus, Lepracanthus.
Immediately above the coal measures lie a variable series of sands and clays of different colours, including the coal plants ; above this, a marl slate in thin layers, containing scanty evidences of fishes ; but these are more abundant and instructive in the superincumbent magnesian limestone, in which formation, near Belfast, ichthyodorulites of the genus Gyrapristis (Ag.) have been found. Above this are the penean red sandstones, in which, at Westoe, have been found fossil spines closely allied to, if not identical with, the Gyracanthus farmosus (Ag.) The foregoing formations constitute the upper most of the palmozoic series called " Permian," from the Russian province in which these strata are most extensively developed. Their relative position is known by the term " magnesian limestone " in the " Table of Strata," fig. 1.
The superimposed strata, marked " new red sandstone," includes also a varied series of red and white sands, marls, and conglomerates, forming collectively the system called " triassic." The ichthyodorulites of this system are referable to the genera Nemacanthus, Leiacanthus, and Hybodus. In the " lias," which is the oldest or lowest of the great " oolitic " system, the dorsal spines of the genus Hybodus (fig. 26), are the largest and most abundant ; this genus, however, is represented by detached teeth in the keuper and muschelkalk members of the " tries." The lias formations give evidence that the dorsal spines and fins of Hybodus were two in number ; and the genus is shown, both by the structure of the spine and the form of the teeth, to have had its nearest affinities with the Cestracion amongst existing Plagiostomes. Hybodus continued to be represented by successive and varying specific forms up to, and including, the cretaceous period. Hybodus is therefore a genus of cartilaginous fishes eminently characteristic of the secondary or mezozoic period in and ranges through every formation of that period. The specimen selected for the illustration of the dorsal spine of Hybodus is that of the H. sub carinatus, from the Wealden of Tilgate Forest.
Large fossil spines, longitudinally grooved, have been found associated with the teeth of the extinct cestrcrciont genus (Ptychodus) of the chalk formations.
In the tertiary formations, the fossil spines present for the most part the generic characters of those of existing Plagiostomes—e. g., Spina.; Trigon, and Mytiobates ; but one form, found in the eocene beds near Paris, is the type of the extinct genus Aulacanthus of Agassiz.
The teeth of the plagiostomous fishes—viz., sharks (Sgua Mee), rays (Raiidce), and Cestracionts, are very numerous, and, being attached only by ligament to the membrane of the month, they must soon fall off in the decomposition of the dead fish, become scattered abroad by the movements of the body through the action of the waters, and sink into the sediment.