no THE LIFE OF THE MOLLUSCA 



gressively dwindle in number, till in the most 

 specialized Septibranchs they are wanting altogether, 

 as they are also, exceptionally, in other less advanced 

 forms (e.g., Anodonta). 



In dealing with the Cephalopoda it is essential to 

 take into account the past history of the race, since 

 so many, especially of the shell-bearing forms, have 

 long been extinct. 



The modern representatives of the class number 

 close on 500 species, belonging to about 80 genera, 

 of which total about half are referable to the genera 

 Polypus, Sepia, and Loligo ; while only 5 species (or 

 less), all belonging to the genus Nautilus, are possessed 

 of an external shell. 



The Nautiloidea, which began in the Cambrian 

 with seven straight - shelled species representing 

 four genera, attained their maximum in the Silurian 

 with about 230 species belonging to 20 different 

 genera and subgenera. Since that epoch they have 

 steadily diminished in numbers down to their 

 minimum at the present day, while the surviving 

 genus, Nautilus, only made its first appearance in 

 the Trias, or in its present limited sense in the 

 Tertiary. Nor did the vigorous offshoot of Am- 

 monoidea that started in the Devonian and attained 

 to a countless host of species, which from some 

 monographs one might almost infer were referable 

 to an equal number of genera, succeed in keeping 

 up the number of testaceous Cephalopoda ; for with 



