534 
DE. A. GUNTHEE’S DESCE1PTI0N OF CEEATODUS. 
forms the base to the attachment of the pectoral spine, which is deeply furrowed, and 
undoubtedly formed by coalescent and stiffened soft fin-rays. As in Lepidosiren and 
Ceratodus, the fin-rays are not joined to the supporting cartilages, but, being disposed 
in two layers, their ends overlap the cartilages. The attachment of the pectoral spine 
to its basal cartilage is not effected by a joint, but by apposition. 
The arrangement in Ceratodus evidently resembles more that in Acipenser than that 
in Polypterus. 
On further inquiry into the more distant relations of the Ceratodus-limb, we may, 
perhaps, be justified in recognizing in it a modification of the typical form of the Sela- 
chian pectoral fin. Leaving aside the usual treble division of the carpal cartilage 
(which, indeed, is sometimes simple), we find that this shovel-like carpal forms the base 
for a great number of phalanges, which are arranged in more or less regular transverse 
rows (zones) and in longitudinal rows (series). The number of phalanges of the zones 
and series varies according to the species and the form of the fin ; in Cestracion pliilippi 
the greater number of phalanges is found in the proximal zones and middle series, all 
the phalanges decreasing in size from the base of the fin towards the margins. In a 
Selachian with a long, pointed, scythe-shaped pectoral fin, like that of Ceratodus, we may, 
from analogy, presume that the arrangement of the cartilages might be somewhat like 
that shown in the accompanying diagram, which I have divided into nine zones and 
fifteen series. When we now detach the outermost phalanx from each side of the first 
horizontal zone, and with it the other phalanges of the same series — when we allow the 
