OF CENTRAL CANADA PART IV. 



277 



deposits known, after Dawson, as the Leda clay formation of the 

 Province of Quebec. 



Cast of Megalnmus (Cyrtodonta) Canademis : 



FIG. 211. 

 Leda truncata. 

 Post Glacial. 



3. IntegripdlUcita : The shell and normal position of the animal 

 in this group are the same as in the Orthoconcha, but the animal 

 possesses a pair of short siphonal tubes. The muscular impressions are 

 connected by a narrow linear depression, marking the edge of the 

 mantel, and this is continuous or without any bend or sinus 

 whence the term " Integripalleata." Tridacna (dating from the 

 Miocene period), Cardium and Lucina (dating from the Silurian 

 period), Cyclas and Cyrena (dating from the Jurassic period), and 

 Crassatella (from the Cretaceous), are some of the more typical 

 genera. The annexed figures represent two of our Palaeozoic forms. 



FIG. 212. 



Lttcina proavia : (Hall). 

 Devonian. 



FIG. 213. 



Conocardium trigonale : (Conrad). 

 Devonian. 



4. Sinupalliata : In this group the animal possesses a pair of 

 long, more or less extensible, siphonal tubes ; and the muscular 

 impressions are connected by a parallel groove with a fold or sinus 

 where the tubes occur. Representatives of the group are hardly 

 known in Palaeozoic strata, but are abundant in Mesozoic and Caino- 

 .zoic formations and in existing seas. Some of the more characteristic 



