THE OLD RED SANDSTONE. 47 
two figures, was submitted to Agassiz during his recent brief 
stay in Edinburgh, and that he as readily recognized in it the 
species of the two kinds which it exhibits, as he had previ- 
ously recognized the species of the originals in the limestone. 
Agassiz, in the course of his late visit to Scotland, found 
six species of the Pterichthys* — three of these, and the 
wings of a fourth, in the collection of the writer. The dif: 
ferences 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 near- 
ly 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 resem- 
bling 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 
* Agassiz now reckons ten distinct species of Pterichthys — P. are- 
natus, 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 hydrophs- 
lus — to the Upper Old Red Sandstone. 
