FINS OF ELASMOBRANCHS. 445 
slightly broader than are the metapterygial radials ; but the bases of the two most post- 
axiad mesopterygial radials have coalesced into a single cartilage. The same is the case 
with the bases of the penultimate and antepenultimate postaxiad radials, also with the 
three next, and again with the two following—/. e. with the seventh and eighth, count- 
ing from the postaxial margin. The most postaxial radial is unsegmented. 
Although the preaxial lobe of the fin, as formed by the fin-rays, extends distally much 
beyond the postaxial lobe, nevertheless the preaxial part of its cartilaginous skeleton 
does not extend distad nearly so much as does its postaxial parts, thus exhibiting a 
marked contrast to the condition presented by Mustelus antarcticus. 
SCYLLIUM CANICULA. 
Dorsa Fin (Plate LX XV. fig. 6). 
This fin has a cartilaginous skeleton, which is separated from the subjacent axial 
skeleton by a wide interval of fibrous membrane. 
The skeleton itself consists of twelve juxtaposed cartilaginous rays, or radials, which 
slope obliquely backwards and upwards, and are all, except the most preaxiad, doubly 
segmented. The segmentation takes place at nearly the same level in all, so that there 
comes to be a triple series of cartilages—one basal, the second median, the third distal. 
The basal cartilages are but ten in number—as the most preaxiad radial has no basal 
cartilage, and as those of the fifth and sixth radials have coalesced to form one large 
cartilage. This large cartilage is also the longest of the series of basals, whence their 
length regularly decreases both pre- and postaxiad. 
There are twelve median cartilages; and all, even the most preaxiad, are longer than 
the longest of the basal cartilages. They increase in length from the first to the sixth, 
and thence slowly decrease. 
The distal cartilages are eleven, since one is appended to the apex of each median 
cartilage except the most proximal one. - They increase slightly in length to the fifth, 
and thence slowly decrease, and are more equal in development than are the basal 
cartilages. Their apices are mostly more or less pointed. There is no coalescence 
amongst them, any more than in the median cartilages. 
Tue CaupaL Fin. 
This fin has inferior cartilaginous supports, which are hemal vertebral processes, 
equal in number to the vertebre whence they spring. Dorsally the fin is supported by 
much smaller but much more numerous cartilages, which have not an exact numerical 
relation to the vertebra, being rather over two to each vertebra. There is here, there- 
fore, as great a contrast between the dorsal and ventral skeletal elements of the caudal 
fin as in Mustelus antarcticus. 
