82 



THE ELASMOBRANCH FISHES 



fig. 85a.) a still more complex type (fig. 86a) has one or two dorsal terminals 

 (d.tr.'^'^) and a ventral terminal (v.tr.), and along the furrow leading to the 

 terminal groove there is an accessor}^ terminal {tr.^), and a dorsal marginal 

 {mg. and d.mg.) {Heterodontus, Mustelus, Scyllium, CarcJiarias, Raja). 



PELVIC GIEDLE 



The pelvic girdle is probably built on a generalized plan in Chlamijdoselachus 

 (fig. 85). Here it is an unusually wide cartilage which is perforated by no 

 fewer than six to eight nerves. In most other types the adult girdle is a narrov/ 

 band which points backward in the middle line. 



Fig. 88. Dorsal fins of Heterodontus francisci, showing fin spines. (Duncan Dunning, del.) 

 A. Second dorsal. B. First dorsal. 



h.c, basal cartilage; ra., radial. 



Figure 87 is a diagram showing the origin of the pelvic girdle as postulated 

 by Thacher (1877). This indicates that the anterior fin radials fuse and the 

 fusion joins a similar fusion from the opposite side to form a bar which be- 

 comes the girdle (pi.) ; the fusion of the tips of the rays back of this becomes 

 the basipterygium (ha.p.). 



SKELETON OF UNPAIRED FINS 



The first indication of the unpaired fins in the embryo of Prisfiurus, Dohrn 

 (1884) found to be the development of a median longitudinal ridge in which 

 cartilage is laid down as a series of parallel rods. In adult Elasmobranchs 

 these cartilaginous rods are usually more or less completely segmented into : 

 (1) basal (h.c, fig. 89), (2) median (h.c.^), and (3) distal segments {h.c.~). 

 The first are proximal or nearest the body axis, and the third often run a con- 

 siderable distance into the fins between the two rows of dermal fin rays. 



In the dorsal fins all these types may be present as single segments of the 

 radials [Mustelus antarcticus, fig. 89a, or Zygaena) . In some forms the distals 



