THE ELASMOBRANCH FISHES 



75 



cyclospondylous type, l)ut the outer circle is usually a thin sheet. Such a type 

 of calcification is known as tecfospofidyly. A modification of the tectospondyl- 

 ous type may add still other outer circles, as, for example, in Squatina. A 

 sagittal section through this type would show the heavy inner circles as V's 

 above and below, which are concentrically surrounded by sections of other 

 circles, appearing as more or less straight lines. 



A most interesting and varied type of calcification is arranged around the 

 inner zone of the notochordal sheath as a central hub from which spokes or 

 rays diverge in a star-like fashion through the outer zone (asterospondyly) . 



A B 



Fig. 81. A aud B. Stages in development of the pectoral fin of Scylliuyn canicula. (From 

 Balfour.) 



ms.p., mesopterjgium ; vit.p., metapterygium; pr.p., propterygium; ra., radial. 



Few calcified rays may be present as in Gal ens, or they may be more numerous 

 as in Heterodontus (fig. 79a) and Rhinohatis (fig. 79b) . In Alopias more than 

 twenty rays are present. 



So characteristic are the above types of calcification that Hasse has used 

 them as a basis for classification. Under such a classification exceptions must 

 be made, however, for many variations are to be found. 



Appendicular Skeleton 



SKELETON OF PAIEED FINS AND OF FIN GIEDLES 



PECTOEAL FIN SKELETON 



The skeleton of the pectoral fin, as noted in Heptanchus, consists of a more or 

 less horizontal framework of cartilage attached to a vertical girdle. The car- 

 tilages making up the framework of the pectoral fin itself are: (1) a set of 

 basal cartilages from which projects (2) a series of median cartilaginous 



