GEOLOGICAL DISTKIBUTION OF GASTEEOPODA. 



369 



Cambrian period, they steadily increase in number, until at the 

 present day the number of known species is far in excess of that 

 recorded from any other period of geological time. According to 

 the estimates made by Bronn between the years 1863 and 1866, 

 which may possibly still serve as a basis for the computation of 

 successive ratios, although no longer abreast of the times, the 

 numerical distribution for the several geological eras is as follows : 



Scaphopoda 



Prosobranchiata 



Heteropoda (inclusive of 

 the BellerophontidsB) . . 



Opisthobraneliiata 



Fulmonata * 



Paleozoic. 



No. of species. 

 22 



141 

 1 



Mesozolc. 



No. of species. 

 48 

 1,764 



162 



Cainozoic. 



No. (»f species, 

 65 

 4,622 



1 

 185 

 630 



Beceut. 



No. of species. 

 60 

 7,500 



64 



825 



6,700 



With very few exceptions (Dentalium, Pleurotomaria, Capulus, 

 Natica, Narica, Emarginula) all the Paleozoic genera are extinct, 

 or at least generally considered to be so, and it is still question- 

 able whether they include even a single siphonate form. The refer- 

 ence of Pusus, Pyrula, and a few other members of the Siphonata 

 to this period, probably rests on unsatisfactory determinations, al- 

 though, indeed, no special reasons can be assigned for the non- 

 extension back of such genera. The absence of prominent characters 

 in many of the species renders their determination diificult or im- 

 possible, and it is by no means improbable that a fair proportion 

 of the genera which have received distinct names are in reality 

 identical with modern genera otherwise designated. The generally 

 holostomate character of the Paleozoic Gasteropoda imparts to the 

 fauna a peculiarity which eminently serves to distinguish it from 

 the similar fauna of the later Mesozoic and Tertiary eras, especially 

 the latter, where the Siphonata largely preponderate. The dominant 

 Paleozoic genera are Pleurotomaria, Murchisonia, Euomphalus, and 

 Loxonema, whose special development throughout the greater part 



* The Pulraonata are now known to be represented by a limited number 

 of species in the Devonian and Carboniferous deposits. The number of 

 Paleozoic species indicated by Bigsby (1868-'78) is about treble the figure 

 given by Bronn. 



