INTRODUCTION 



THERE LIVES today a vast group of fishes, some of which are littoral, keep- 

 ing close to shore; others are the nomads of the ocean, roaming vast ex- 

 panses of its waters; others there are which are pelagic, living near its 

 surface; and still others that are the inhabitants of the profound depths into 

 which sunlight never penetrates — these are the sharks, to the man with nets the 

 most worthless, to the naturalist among the most interesting of living things. 

 But the vast numbers of today are few in comparison with the hordes which 

 have lived and passed in succession before them. They, the rulers of the waters 

 in bygone ages, have gone down like primitive man, leaving little to tell of 

 their presence. This little, however, is of singular interest. Some of these fishes 

 are known to us only by a spine. Others are represented by ])its of armament 

 which show that many of the ancient sharks were clad in a protective covering 

 far more complex than that possessed by any of the living forms. But most of 

 these fishes are known to us by their teeth; of these, the heterodont sharks are 

 most instructive. 



Before me are the teeth of a form which swam the primitive seas before the 

 formation of our western mountains. Beside them I place the teeth of another 

 wdiich was taken with hook and line in the Pacific but yesterday. The vast 

 stretch of years separating the life of the one from the life of the other is be- 

 yond the comprehension of man, yet the close similarity of plan binding the 

 one form to the other clearly indicates that this of the present is that of the 

 past projected through the ages. 



Not upon fragments alone does our information concerning these ancient 

 fishes rest. Within the past few decades our knowledge has been greatly en- 

 riched by the discovery of specimens, many parts of which were in an almost 

 perfect state of preservation. This preservation is the more wonderful when it 

 is recalled that even the harder cartilaginous parts are subject to rapid decay. 

 That some of these specimens have escaped the ravages of time makes us hope- 

 ful that others of still more archaic forms will yet be unearthed, to complete 

 our records of the ancient history of this group. 



Of the extinct types discovered in an excellent state of preservation, Clado- 

 selachus (fig. 10), described by Dean, is one of the oldest and in many ways 

 one of the most generalized and interesting of fishes. From it we have learned 

 much ; for even to detail, soft parts like muscle fibers and, in some specimens, 

 visceral organs have been obtained in a remarkable state of preservation. Two 

 other forms, Acanthodes and Pleur acanthus, are of special interest. In an 

 acanthodian type like Cliniatius{ ?) (fig. 11) spines have been developed to an 

 unusual degree so that even in the paired fins the fin is composed essentially 

 of web and anterior supporting spine. In this type the spine is essentially a 

 dermal structure encasing the exposed margin of the fins. In other regions of 

 the body the scales show the same tendency toward hypertrophy. Thus around 

 the eye and along the lateral line they are of unusual size. In Pleur acanthus, 



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