Genus Gyracauthus, Agassiz. 43 
rately to what elements of the Selachian skeleton Messrs. 
Hancock and Atthey compare them. 
The term “carpal” is not used by anatomists of the 
modern school to denote any part of the skeleton of the fore 
limb in fishes; but on turning to Prof. Owen’s ‘ Compara- 
tive Anatomy of the Vertebrata,’ vol. 1. p. 168, fig. 104, we 
find the three basal cartilages of the pectoral fin of the picked 
dogfish so designated. ‘Two of these, the mesopterygium and 
metapterygium of Gegenbaur, are triangular, with their apices 
directed towards the shoulder-girdle, while the third or pro- 
pterygium has an oblong shape, faintly reminding us of the 
second form of so-called carpal of Gyracanthus. There can 
thus be no doubt that these basal cartilages, which, in the 
skeleton of the recent shark, intervene between the shoulder- 
girdle and the radial cartilages, or cartilaginous fin-rays, are 
the elements which Messrs. Hancock and Atthey meant by 
the term carpal. And the question is simply this, Is it likely 
that the process of calcification in such cartilages would give 
rise to bodies like the peculiar bones so often -found associ- 
ated with the spines of Gyracanthus? Or can any better ex- 
planation of their nature be suggested ? 
One point in their external configuration was not noticed 
by Messrs. Hancock and Atthey, namely, that these bodies 
were hollow, and that their extreme flatness is due to the 
crushing together of the thin walls of the internal cavity. If 
we take first one of the triangular series, it may easily be seen 
that the two walls, or laminz of which the bone is composed, 
are united at the apex and along the two thick sides which 
meet at the apex, but that they are separate at the thin base, 
at which accordingly the cavity was open. It may also be 
seen that the edges of the basal opening do not coincide, as 
careful development of these edges shows that the one on the 
convex side of the bone is indented by a large angular notch 
or sinus, which runs up for some distance in the direction of 
the apex; this appearance I have seen in every case in which 
I have looked for it. The internal cavity is at once distin- 
guishable, filled with matrix, when a specimen is broken or 
cut across. I have equally assured myself of the hollow 
character of the bones of the second series. 
If we now look at the texture of these bodies we shall be 
at a loss to explain the expressions “ imperfectly ossified ” 
and “ semicartilaginous,”’ used by Messrs. Hancock and 
Atthey, in the passages already quoted. On examining the 
surface with a lens its apparent fibrous aspect is seen to be 
due to its being closely covered with minute grooves inter- 
spersed with small openings, these markings being clearly 
