46 THE LIFE OF THE MOLLUSCA 
first known Chiton (Priscochiton). ‘The Cephalopoda 
‘had increased in number to sixty-five species of 
Nautiloidea, the majority still being straight-shelled, 
but some curved and a few coiled forms (the first) 
are included. 
With the Silurian epoch a considerable increase 
in the number of Mollusca becomes evident. To 
the Gastropods are added members of the families 
Trochidz, Epitoniide, and Xenophoride. Among 
Pelecypods, Palzeoconchs were most abundant, but 
all orders save the Septibranchs were represented. 
Two more Chitons (Helminthochiton and Chelodes) 
made their appearance, and shells that have been 
referred, though doubtfully, to Scaphopoda. The 
most marked feature of the epoch, however, was the 
abundance of the Nautiloidea, which then attained 
their zenith with about 230 species, among which 
coiled were almost as abundant as the other shell 
forms. From that day the group has steadily 
diminished in numbers, only five species now exist- 
ing, or as some reckon them, fewer still. 
The Devonian strata have yielded evidence of the 
further increase in the Pelecypods, representatives of 
the Filibranch families—Trigoniide, Pectinide, and 
Mytilida —appearing with the Eulamellibranch 
families—Pinnide, Cardiniide, Megalodontide—and 
such specialized forms as Pholadella and Allorisma, 
the latter being the earliest example of a Pelecypod, 
showing clear evidence of retractile siphons. Of 
