FINS OF ELASMOBEAJN'CHS. 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 — i. 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. 



DoKSAL Fin (Plate LXXV. 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 hasal 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. 



The Caudal Fin. 



This fin has inferior cartilaginous supports, which are haemal vertebral processes, 

 equal in number to the vertebrae 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 vertebrae, 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. 



