OCR Text |
Show 38 FISHES ABUNDANT creatures, zoophyta, polypiaria, conchifera, crustacea; but to these are added numerous fishes, some of which are of the most extraordinary and surprising forms. Several of the strata are crowded with remains of fish, showing that the seas in which those beds were del?osited had swarmed with that class of inhabitants. The Investigation of this system is recent ; but already* M. Agassiz has acertained about twenty genera, and thrice the number of species. And it is remarkable, that the Silurian fishes are here only represented in genera; the whole of the species of that era had already passed away. Even throughout the subgroups of the system itself, the species are changed; and these are phenomena observed throughout all the subse .. quent systems of geological eras; apparently arguing that, during the deposition of all the rocks, a gradual change of physical conditions was constantly going on. A varying temperature, or even a varying depth of sea, would at present be attended with similar changes in marine life · and by analogy, we are entitled to assume, that such vari~ ations in the ancient seas might be amongst the causes of that constant change of genera and species in the inahabitants of those seas, to which the organic contents of the rocks bear witness. Some of the fossils of this system-the cephalaspis, coccosteus, pterichthys, holoptychius~are, in form and structure, entirely different from any fishes now existing, only the sturgeon family having any trace of affinity to them in any respect. They seem to form a sort of connecting link between the crustacea and true fishes. The cephalaspis may be considered as making the smallest advance tl·om the crustacean character; it very much resembles in form the asaphus of lower formations, having a longish tail-like body inserted within the cusp of a large crescent-shaped head. somewhat like a saddler's cuttingknife. The body is covered with strong plates of bone, enamelled, and the head was protected on the upper side \Vith one large plate, as with a buckler-hence the name, implying buckler-head. A range of small fins conveys the idea of its having been as weak in motion as it is strong in structure. The coccosteus may be said to n1ark the next ad vance to fish creation. The outline of its body is of the form of a short thick coffin, rounded covered with strong bony plates, and terminating in a long tail • June, 1842. ERA OF THE OLD RED SANDSTO:rfE 39 ~Lich seems to have been t~e sole organ of motion It lS very remarkable, that, w lule the tail establishes this creature among the vertebrata and the fishes its mouth has bee_n orened vertically, like those of the ~rustaceans, but_ \Vluch IS contrary to the mode of vertebrata generally. Th1s seems a pretty strong mark of the link character of t~e co?coste':ls between these two great departments of t.ne arumal k~ngdom. The pterichthys has also strong bony pl~tes over Its body, arranged much like those of a tortOise, and h~s a long tail ; but its most remarkable feature, and that whic~ has suggested its name, is a pair of long and narr?W-wing like appendages attached to the shoulders, which the creature is supposed to have erected for its defence when attacked by an enemy. The holoptychius is of a flat oval form, furnished with fi~s, and ending in a long tail; the whole body covered With stro~g plates, which overlap each other, and the head forming only a slight rounded projection from the general figure. The specimens in the lower beds are not _above the size. of a flounder; but in the higher strata, to Judge by the size of the scales or plates which have been fo~nd, the creature att_ained a comparatively monstrous size. 1~he o.ther fishes of the system-the osteolepis, glyptolepls, d! pter~s,. &c., are, in general outline, much like fishes still ex1sbng, but their organization has, neverthel~ ss, some striking peculiarities. They have been entirely covere~ with bony scales or plates, enamelled extP. r_n~lly; then· spines are tipped with bone, anrl, as one stn1nng and unvaryin()' feature the tail is only finned on the 1 . ~ ' ower side. The internal skeleton of which no traces have been preserved, is presumed to' have been cartilaginous. ~.,hey therefore unite the character of cartilaginous fish.es With a character peculiar to themselves; and in _whiCh we see pretty clear vestiges of the pre-existent crustaceous form. With regard to the link character of these animals. ~orne curious facts are mentioned. It appears that in the r~npe~fect .condition of the vertebral column, and the inferiOr sttuatw_n of the mouth in the pterichthys, coccosteus, &c., th~r~ 1s an analogy to the form of the dorsal cord tmd pos1t1?n of the mouth in the embryo of perfect fishes. Th.e ~ne-sided form of the tail in the osteolepis, &c., finds \ Similar analogy in the form. of the tail, in the embryo of |