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 

 Trochidse, Epitoniidas, and Xenophoridse. Among 

 Pelecypods, Palaeoconchs were most abundant, but 

 all orders save the Septibranchs were represented. 

 Two more Chitons {Helminthochiion 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 — Trigoniidae, Pectinidae, and 

 Mytilidae — appearing with the Eulamellibranch 

 families — Pinnidse, Cardiniidae, Megalodontidse — and 

 such specialized forms as Pholadella and Allorisma, 

 the latter being the earliest example of a Pelecypod, 

 showing clear evidence of retractile siphons. Of 



