MINERALS AND GEOLOGY OF CANADA. 



117 



talves are right and left, instead of upper and 



under, as regards the normal position of the 



animal, and the muscular impressions are two 



in each valve. The fossil species known as 



Modiolopsis modiolaris fig. Ill, so common in 



our Hudson River Group, belongs in all pro- 

 bability to this division. The genus Cyrto- 



donta of Billings, (with its sub-genus Vanuxe- 



mia), may also be referred to the Orthoeoncha of 



this Section. Fig. 112 represents the Cyrto- 



donta JSuronensis (var. siibcarinata) of the 



lower part of the Trenton Group. Another 



and more remarkable species of this genus — 



widely known as the Megalomus Canadensis, of 



Hall — occurs in great numbers in the Onondaga Salt Group, (Upper 

 Silurian), of Canada West, and more especially 

 in the neighbourhood of Gait. It is found 

 chiefly in the form of internal casts, as shewn 

 in the figures 113 and 113 a. 



The lamellibrancJis of the third group, IntB' 

 gro-PaLlialia, have the upright (or right and 

 left) position of the orthoeoncha of Section I., 

 but, unlike these latter, they possess a pair of 

 short * respiratory tubes. The muscular im- 

 pressions, two in each valve, are connected, as in the forms of the 



Fig. 111. 



Fig. 112. 



Fig. 113. Fig. 118. a. 



last group, by one uninterrupted shallow groove or " pallial impres- 

 sion," — i.e., a continuous line without any bend or sinus in it. The 

 existing fresh-water genus Cyclas, species of which occur in our 



