IIo 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 (¢.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 
