2 THE ELASMOBRANCH FISHES 



there are many of the present-day characteristics peculiar to the sharks. In 

 addition, bj' the structure of its limbs and tail, it suggests relationship to the 

 interesting group of Dipnoans or lungfishes. 



Equallj' as interesting as the extinct forms preserved to us in the rocks are 

 others, less ancient though the}' be, which have come down to us in flesh and 

 blood. Of these living representatives of the past some are among the most 



Fig. 10. Cladoselachus. (From Dean.) 



interesting and instructive of forms, interesting as antiquities are interesting, 

 instructive as all generalizations are instructive. Among these forms known to 

 us on the California coast are Heterodontns francisci (fig. 17), whose genus is 

 the sole survivor of a group, twenty-five genera of which are buried in the 

 rocks; and Hexanchus corinus and Heptanch us maculatus (fig. 13) , the latter 



Fig. 11. Climatius(^). (From Dean.) 



with generalization of bodily plan surpassing that of any other present-day 

 Elasmobranch. 



In addition to the ancient types there are many modern forms. Some of 

 them, like Acanthias (fig. 5, Squalus acanthias) , have system upon system so 

 generalized as to approximate a ground plan on which nature has built up its 

 masterpieces of vertebrate life. Other forms, although simple in part, are 

 noted for extreme specialization in certain respects. Among these may be men- 

 tioned Cephaloscyllium, the California swell shark (fig. 1), Zygaena, the 

 hammer-head common to many seas, and Alopias, the thresher, a single genus 

 of world-wide distribution (fig. 2). 



Still others of the recent sharks, although exceeding Zygaena and Alopias 

 but slightly in size, are singled out as large sharks. Of this group the forms 

 known to occur on the Pacific Coast are CarcJiarias (Prionace) , the "man- 



