118 



A POPULAR EXPOSITION OF THE 



Post-Tertiary deposits, and especially in those of Western Canada, 

 may be cited as an example of the present group. (See Paet V.) 



Finally, the moUusks of the fourth group, Sinu- Palilalia, possess 

 a pair of long siphonal-tubes, capable of extension beyond the shell ; 

 and their two muscular impressions are united by a more or lesa 

 deeply sinuated pallial line. Many of these lameUibranchs burrow 



Fig. 115. 



Fig. 114. 



in the sand of the shores on which they live, between the tide-marks> 

 with their respiratory tubes extending to the surface ; and fossil 

 examples occupying this upright position, and thus shewing the 

 animals to have been fossilized in their original burrows, are met with 

 in certain strata. As examples of the group, we may refer to Mya 

 truncata, fig. 114, and to Saxicava rugosa, fig. 115, both of which are 

 of exceedingly common occurrence in the Post-Tertiary deposits of 

 Eastern Canada. 



Fteropoda : — The living pteropods are swimming or floating mol- 

 lusks, frequenting the open sea. Some few are naked, but the 

 greater number secrete a delicate external shell (univalve,) and all 

 possess a pair of fins or wing-like appendages for natatory purposes. 

 In the pteropods with shells, the head is more or less indistinct. 

 The Oonularia is the only form of Canadian occurence, referrible, and 

 that doubtfully, to this class. Fig. 116 represents 

 0. Trentonensis of the Trenton Group. The shell in 

 this genus is more or less conical and four-angled, 

 furrowed longitudinally, and marked transversely by 

 numerous straight or zig-zag lines. These latter 

 often resemble rows of minute punctures. The 

 genus extends from the Lower Silurian division into 

 the Lias formation of the Mesozoic rocks. Fig. iie. 



Heteeopoda. — The representatives of this class are regarded by 

 many naturalists as forming simply an Order (N'ucleohranchiata) of 



