

90 STUDIES ON THE ELASMOBRANCH SKELETON, 



basibranchial plate is broad and leaf-like in front, while behind it 

 is produced into a rather slender process. This arrangement of 

 the copulae differs from that observed in Heptanchus cinereus as 

 described by Gegenbaur* in the absence of an independent 

 copula for the fifth arch. In the first to the fifth branchial arches the 

 basals of opposite sides touch one another in the middle dorsal 

 line, the basals of the first being united with one another by- 

 cartilage. The basals of the sixth do not reach to the middle line. 

 There are small but distinct muscular grooves on the dorsal and 

 ventral mesial cartilages. 



The external branchial arches are represented only by two pairs of 

 cartilages in each ; the one is connected with the external end of 

 the basal ; the other, which is much longer, with the ventral end of 

 the ventral cartilage. The free ends of these two slender cartilages 

 bend round the outer edge of the gill-partition towards one another, 

 but do not meet. These obviously represent modified rays, and 

 their presence would seem to indicate that the more highly 

 developed external branchial arches of other Selachians are 

 derived from greatly elongated rays which ultimately lose their 

 connection with the internal arches. 



The Pectoral Fins. 



The pectoral fin is very similar to that of Hexanchus griseus as 

 described and figured by Gegenbaur. The propterygium is small 

 and beai-s no rays, articulating distally with the mesopterygium. 

 The mesopterygium and the metapteiygium bear a nearly equal 

 number of rays, a few of them being bifurcate. The metap- 

 terygium beai-s a small articular cartilage at its extremity, and the 

 latter sustains about four rays. 



The shoulder-girdle is remarkable for the presence in the middle 

 ventral line of a distinct four-sided lozenge-shaped cartilage let in 

 to the arch, as it were, in front. This is a condition which 

 I have not observed or seen described in any other form : it 

 does not seem to occur either in Heptanchus cinereus or 



* 1 c. p. 136, pi. XVIII., fig. 1. 



