THE ELASMOBRANCH FISHES 



81 



There is a foraiiieii enterino- flie median side of the girdle for the brachial 

 artery and for nerves going to the pectoral fin. The canal leading through the 

 girdle from tliis foramen sei)arates into a dorsal and a ventral part so that 

 laterally there are two foramina leaving the girdle. 



The coracoids from the opposite sides may be separated by a special un- 

 paired sternal piece (HepfdncJuis maculotus). Usually, however, they are 

 joined ventrally in the sharks; in Heterodontus and Squatina they are firmly 

 welded together. In the rays this region is firm excepting in Torpedo. 



PELVIC FIN SKELETON 



The skeleton of the pelvic (ventral) fin is made up of at least two basal car- 

 tilages, the basipterygium (ha. p., figs. 85-86) and the anterior basal. From 



Fig. 87. Diagram to illustrate the probable origin of the pelvic girdle. (From Mivart 

 after Thacher.) 



ha.p., basipterygium; pL, pelvic girdle; ra., radial. 



these two basal pieces the radials proceed. The basals are supported by a girdle 

 w^iich in the Elasmobranchs is not in contact with the axial skeleton. The 

 structure of the adult pelvic fin is much simpler than that of the pectoral, but 

 in the embryo the two are built on the same fundamental plan. The basipteryg- 

 ium (ha.p.) is comparable to the metapterygium of the pectoral. This is 

 normally a single piece, but posteriorly it may be broken into three or four 

 segments (Centrophorus, Heterodontus, fig. 86a ; Rhinohatis, fig. 86b). The 

 anterior basal is somewhat like the propterygium. It apparently represents a 

 fusion of the basal parts of the anterior radials, from the distal part of which 

 the radials extend freely. In Heptanchus (fig. 55) a segment may join the 

 girdle between the anterior basal and the basipterygium which in position is 

 like the mesopterygium. The radials belonging to the basipterygium proper 

 are more or less segmented anteriorly, but posteriorly, in both male and 

 female, they are usually unsegmented. 



The skeleton of the clasper of the male is a continuation in the median axis 

 of the basipterygium of the fin, the two being connected by short segments 

 {h.^ and h.^, fig. 86a) as in Heptanch us. The terminal part of the basal or axial 

 cartilage assumes dififerent degrees of complexity in the various Elasmo- 

 branchs. In some, the basal or axial cartilage (ha.) is provided with a single 

 accessory cartilage. In others there are present distally an outer marginal 

 and a ventral and a dorsal terminal accessory cartilage {Chlamydoselachus, 



