148 University of California Publications in Zoology [Vol. 13 



A. INTRODUCTION 



From an anatomical point of view, the elasmobranch fishes have 

 long been of interest. Indeed, nearly a century ago Rathke, as well 

 as Cuvier, in studies on these fishes made the observation that they 

 possess a remarkable generalization of bodily plan. Later, through 

 the notable researches of Gegenbaur, Huxley, Balfour and others, the 

 plan came to be considered by many as the foundation upon which 

 Nature has reared her masterpieces of vertebrate life. 



The value of an understanding of any type, primitive in character, 

 can hardly be overestimated ; for, in addition to the knowledge gained 

 of the type itself, there is often made possible the interpretation of 

 structures and systems found in higher forms. In this respect a study 

 of the elasmobranch fishes has been particularly instructive. It is 

 upon these that we have gained wholly or in part our conception of 

 some of the profound problems of biology. Among such problems 

 may be mentioned the origin of vertebrate teeth, the evolution of 

 paired limbs, and the nature of the skull. 



Since the work of the earlier anatomists, the generalization of plan 

 so evident and characteristic of some orders of the elasmobranch group 

 has been found to give place to specialization in a different family 

 of the same order. As such may be mentioned the recently described 

 notidanid shark, Pentanclms, with five gill-clefts instead of the more 

 generalized number characteristic of other members of the notidanid 

 group (Smith, 1912, and Regan, 1912). In fact, it has been indicated 

 that transition from a more generalized plan to one more highly spec- 

 ialized may take place within the lifetime of a single individual. An 

 example of such may be found in Heterodontus, the embryo of which 

 possesses rudiments of a sixth gill arch, only five arches becoming 

 functional in the adult (see Hawkes, 1905). 



It has been my privilege to make investigations on another of the 

 heterodont sharks, Heterodontus francisci, found on the California 

 coast. Realizing the interest pertaining to the heterodont group, I 

 have studied somewhat in detail several of its major systems. The 

 first of these I shall now consider. 



