Agassiz, in the course of his late visit to Scotland, found six species of the _Pterichthys_[J]--three of these, and the wings of a fourth, in the collection of the writer. The differences by which they are distinguished may be marked by even an unpractised eye, especially in the form of the bodies and wings. Some are of a fuller, some of a more elongated, form; in some the body resembles a heraldic shield, of nearly the ordinary shape and proportions; in others the shield stretches into a form not very unlike that of a Norway skiff, from the midships forward. In some of the varieties, too, the wings are long and comparatively slender; in others shorter, and of greater breadth: in some there is an inflection resembling the bend of an elbow; in others there is a continuous swelling from the termination to the shoulder, where a sudden narrowing takes place immediately over the articulation. I had inferred somewhat too hurriedly, though perhaps naturally enough, that these wings, or arms, with their strong sharp points and oar-like blades, had been at once paddles and spears--instruments of motion and weapons of defence; and hence the mistake of connecting the creature with the Chelonia. I am informed by Agassiz, however, that they were weapons of defence only, which, like the occipital spines of the river bull-head, were erected in moments of danger or alarm, and at other times lay close by the creature's side; and that the sole instrument of motion was the tail, which, when covered by its coat of scales, was proportionally of a somewhat larger size than the tail shown in the print, which, as in the specimens from whence it was taken, exhibits but the obscure and uncertain lineaments of the skeleton. The river bull-head, when attacked by an enemy, or immediately as it feels the hook in its jaws, erects its two spines at nearly right angles with the plates of the head, as if to render itself as difficult of being swallowed as possible. The attitude is one of danger and alarm; and it is a curious fact, to which I shall afterwards have occasion to advert, that in this attitude nine tenths of the _Pterichthys_ of the Lower Old Red Sandstone are to be found. We read in the stone a singularly preserved story of the strong instinctive love of life, and of the mingled fear and anger implanted for its preservation--"The champions in distorted postures threat." It presents us, too, with a wonderful record of violent death falling at once, not on a few individuals, but on whole tribes. [Footnote J: Agassiz now reckons ten distinct species of _Pterichthys_--_P. arenatus_, _P. cancriformis_, _P. cornutus_, _P. major_, _P. Milleri_, P. latus, _P. oblongus_, _P. productus_, _P. testudinarius_, and _P. hydrophilus_; of these, nine species belong to the Lower, and one--the _Pterichthys hydrophilus_--to the Upper Old Red Sandstone.] [Illustration: _PLATE III._]