OF THE EliASMOBBANCHS. 



73 



those now inhabiting European seas. According to Dr. Giinther's Catalogue of the 

 Fishes in the British Museum, Vol. viii, there are now living only seventy-six European 

 species. During the Miocene there existed one hundred and thirty-five species in that 

 region. In North America, the recent species exceed those known from the Miocene in 

 the ratio of eighty-six to fifty-seven ; but if only those found along our eastern and 

 southern coasts are compared with the Miocene species of the same region, the numbers 

 are about equal. Doubtless, when our marine Miocene fauna is hotter known, especially 

 that of our western coast, the number of species will he increased. When all the Mio- 

 cene species of Europe and America combined are compared with all the living species 

 of the same region, we find the numbers to he approximately 155 and 135. No reason 

 is apparent why these two great regions should not represent well the state of Elasmo- 

 branchian development during Miocene times ami thai of to-day; and even it we 

 leave; out of account the insufficiency of the geological record, it shows that, as regards 



i ibers of species, Ibis class has declined. 



The remarkable reduction in the number of genera and species of Elasmobranchs 

 during the Permian period finds a parallel in the apparently almost complete extinction 

 of the Brachiopoda at the same time. The lads hearing on this subject have been 

 obtained from Mr. Charles Schuchert's Synopsis of the Brachiopoda of North America. 

 The following table shows the number of genera which are known to have existed at 

 each geological age : 



Cambrian 28 



Ordovician CO 



Silurian 75 



Devonian 101 



Carboniferous 7<> 



Triassic... 

 Jurassic .. . 

 Cretaceous 

 Tertiary. • ■ 



48 

 48 

 81 



20 



This table has the appearance of proving that the Brachiopoda culminated during 

 the Devonian age, thereafter began to decline, and have continued to diminish gradually 

 to the end of the Tertiary; but if we inquire a little more closely into their chrono- 

 logical distribution we shall find that in the Permian deposits there have been found 

 only eight genera and eight species, as Schuchert has shown. Therefore, a curve that 

 would represent the history of the Brachiopoda would resemble closely that representing 

 the history of the Elasmobranchs, except that many phases of the former curve! would 

 fall about out! age earlier than in the case of the latter. 



Concerning the other families of post-palseozoic sharks, I have no remarks to make. 

 The group of rays will be briefly considered. The question of their origin assumes 

 importance from the discovery of Tamiobatis, described by Dr. C. R Eastman.* The 



; Amw. Jour. Sol, iv, 1807, p. 85. 



