Strange as seem the forms and structure of the placo ganoid fishes of the " old red " period, there are not wanting existing species which throw much truer light on their nature than any existing Ch,e,lonia or Crustacea. The singular little family of " trunk-fishes " (Ostracionidoe) shows species in which the body is inclosed in a more or less quadrangular cuirass, composed of suturally-articulated ganoid plates, which are usually tuberculated on the external surface, and with the angles prolonged into spines in some species, like those of the helmet of Cephalaspis. The caudal part of the trunk protrudes from the back opening of the cuirass, as in Coccos teus and Pterich.thys, and ossification of the endo-skeleton is incomplete. The species of this family are for the most part natives of seas of tropical or warm temperate latitudes.
In another family of existing fishes, called " Siluroids," there are species in which the broad cranial bones, connate with dermal ossifications, form a helmet to the head, whilst one or two dermal spine-bearing bones combine to form the part called " buckler " by In the genus Doras, the lateral line is armed with bony ganoid plates ; and in Callich. thys, these biserial plates are developed so as to incase the whole body. But generally, as in Pimelodus, the hinder muscular part of the trunk is undefended, as in Coccosteus. The ganoid plates of the head and back shields are fretted with rows or ridges of confluent tubercles, radiating from the centre to the circumference of the plate, whilst the inner surface is smooth, as in Coccosteus (fig. 46) ; and, moreover, the dorsal plate in existing Siluroids sends down a median ridge from its inner surface, like that from the " dorso-median " plate in Coccosteus. The point of resemblance to be mainly noticed, however, is the contrast furnished by the powerful armature of the head and back with the unprotected naked ness of the posterior portions of the creature—a point specially noticeable in Coccosteus, and apparent also, though in a lesser degree, in some of the other genera of the old red, such as the Pterichthyes and Asterolepides. "From the snout of the Coccosteus down to the posterior termination of the dorsal plate the creature was cased in strong armour, the plates of which remain as freshly preserved in the ancient rocks of the country as those of the Pimelodi of the Ganges on the shelves of the Elgin museum ; but from the pointed termination of the plate immediately over the dorsal fin to the tail, com prising more than one-half the entire length of the animal, all seems to have been exposed, without the protection of even a scale ; and there survives in the better specimens only the internal skeleton of the fish and the ray-bones of the fins. It was armed, like a French . dragoon, with a strong helmet and a short cuirass ; and so we find its remains in the state in which those of some of the soldiers of Napoleon's old guard, that had been committed unstripped to the earth, may be dug up in the future on the fatal field of Borodino, or along the banks of the Dwina or the Wap. The cuirass lies still attached to the helmet, but we only find the naked skeleton attached to the cuirass. The Pterichthys to its strong helmet
and cuirass added a posterior armature of comparatively feeble scales, as if, while its upper parts were shielded with plate-armour, a lighter covering of ring or scale armour sufficed for the less vital parts beneath. In the Asterolepis the arrangement was somewhat similar, save that the plated cuirass was wanting. It was a strongly-helmed warrior in slight scale-armour ; for the disproportion between the strength of the plated head-piece and that of the scaly coat was still greater than in the Pterich,thys. The occipital star-covered plates are, in some of the larger specimens, fully three quarters of an inch in thickness, whereas the thickness of the delicately-fretted scales rarely exceeds a line.
" Why this disproportion between the strength of the armature in different parts of the same fish should have obtained, as in Pterichth,ys and Asterolepis, or why, while one portion of the animal was strongly armed, another portion should have been left, as in Coccosteus, wholly exposed, cannot of course be determined by the mere geologist. His rocks present him with but the fact of the disproportion, without accounting for it. But the natural history of existing fish, in which, as in the Pimelodi, there may be detected a similar peculiarity of armature, may perhaps throw some light on the mystery. In Hamilton's Fishes of the Ganges, the habitats of the various Indian species of Pimelodi, whether brackish estuaries, ponds, or rivers, are described, but not their charac teristic instincts. Of the Silurus, however, a genus of the same great family, I read elsewhere that some of the species, such as the Sawrus Glanis, being unwieldy in their motions, do not pursue their prey, which consists of small fishes, but lie concealed among the mud, and seize on the chance stragglers that come in their way. And of the Pimelodus gulio, a little strongly-helmed fish with a naked body, I was informed by Mr. Duff, on the authority of the gentleman who had presented the specimens to the Museum, that it burrowed in the holes of muddy banks, from which it shot out its armed head, and arrested, as they passed, the minute animals on which it preyed. The animal world is full of such com pensatory defences ; there is a half-suit of armour given to shield half the body, and a wise instinct to protect the rest. Now it seems not improbable that the half-armed Coccosteus, a heavy fish, indifferently furnished with fins, may have burrowed, like the recent Silurus Glanis or Pimelodus gulio, in a thick mud, of the existence of which in vast quantity, during the times of the old red sandstone, the dark Caithness flagstones, the foetid breccia of Strathpeffer, and the gray stratified clays of Cromarty, Moray, and Banff unequivocally testify ; and that it may have thus not only succeeded in capturing many of its light-winged contemporaries, which it would have vainly pursued in open sea, but may have been enabled also to present to its enemies, when assailed in its turn, only its armed portions, and to protect its unarmed parts in its burrow."*