460 



ELASMOBRANCHII 



[CH. 



Limbs. 



tail-fin, which is now denominated heterocercal, since the axial 

 skeleton does not divide it into two equal parts (Fig. 227). 

 The paired fins are attached to hoops of cartilage (the limb 

 arches), called respectively the pectoral and pelvic 

 girdles, the pectoral being situated just behind the 

 last gill-cleft, the pelvic just in front of the anus. The pectoral 

 girdle extends a considerable distance up the side of the animal: 

 the pelvic is little more than a transverse bar. The fins in modern 

 Elasmobranchs are of what is called the uni seriate type, that is 

 to say, there is a thick jointed main axis with cartilaginous rays 

 attached only to its anterior border. Fossil Elasmobranchs show in 

 one case, Pleuracantkus, abiseriate fin with rays attached to both 

 borders ; and in another, Cladoselacfie, a still more primitive con- 

 dition, where the fin is merely a lateral flap supported by parallel 

 bars of cartilage. By the coalescence of these at the base the axis 

 was formed, and later by the disappearance of the rays on one side, 

 the uniseriate fin. In Cladoselache there seems to have been no 

 limb girdle. This is a newer development and must be regarded 

 as a strengthening of connective tissue at the base of the fin in 

 order to provide a firm insertion for the muscles moving the fin. 

 In the pectoral fin the basal portions of some of the rays 

 coalesce to form two large cartilages 

 called propterygium and meso- 

 pterygium, whilst the axis itself is 

 called the metapterygium. In the 

 pelvic fi.n of the male the axis bears 

 distally a grooved rod which is termed 

 the clasper, and is used in transfer- 

 ring spermatozoa to the female. The 

 axis is called the basipterygium. 

 The distal joints of the rays in both 

 pectoral and pelvic fins are made up 

 of numerous small cartilages called 

 radialia. 



The brain of Elasmobranchs is re- 

 markable for the great 

 development of the ol- 

 factory lobes, which are in close contact 

 with the nasal sac and are attached 

 by a narrow stalk to the cerebrum. 

 This is only imperfectly divided into 



FIG. 225. Dorsal view of the 

 pelvic girdle and fins of a 

 male Dog-fish, ScylUum cani- 

 cula. From Reynolds. 



1. Pelvic girdle. 2. Basi- 

 pterygium. 3. Clasper. 

 4. Radialia. 



Brain. 





