276 



small impression whence the terms Monomyarla and Heterjmyaria, 

 often applied to the members of this group. Some of the more 

 common genera comprise : Ostrcea (dating from the Carboniferous 

 period) ; Spondylus (dating from the Triassic period) ; Pecten (dating 

 from the Devonian period) ; Lima (dating from the Carboniferous 

 period); Avicula (dating from the Silurian period) ; Mytilus (dating 

 from the Triassic period) ; and Pinna (dating from the Devonian 

 period), Among the most frequent forms of Palaeozoic age, occurring 

 in our strata, Ambonychia radiata (Family Aviculidce), and Modio- 

 lopsis modiolaris (Family Mytilidce) may be especially cited. 



FIG. 209. 



Modiolopsis modiolaris. 

 Hudson River Formation. 



FIG. 208. 



Ambonychia radiata. 



Trenton and Hudson 



River Formations. 



2. Orthoconcha : This group, so named by the French palaeonto- 

 logist Alcide d'Orbigny. is the Homomt/aria of some authors. The 

 valves of the shell in the normal position of the animal tir right and 

 left,* and the muscular impressions are two in number and of prac- 

 tically equal size. In these respects, therefore, the Orthoconcha are- 

 more closely allied to the Siphonida Can to the Pleuroconcha^ 

 although they are without siphonal tubes, properly so-called. The 

 marine forms Area, Nucula and Leda (all dating from the Silurian 

 period), Trigonia (dating from the Liassic period), and the fresh- 

 water UniOy are some of the more representative genera. The fossil 

 genus Megalomus or Cyrtodonta probably belongs to the Arcacece. 

 One species, M. Canadensis, found mostly in the form of casts of 

 comparatively large size, is very characteristic of the (Upper Silurian) 

 Guelph formation. Other species (Cyrtodonta of Billings) are of 

 common occurrence in the Hudson River formation. A species of 

 Leda (L. truncata) is equally characteristic of the Post-Glacial 



* The conventional position adopted in drawings does not represent the normal position 

 the living lamellibranchiates. 



