278 



MINERALS AND GEOLOGY 



genera comprise : Venus, Cytherea, Tellina, and Mactra (all dating 

 from the Jurassic period), Donax and Solen (both dating from the 

 Eocene) and Mya, Saxicava, &c. (dating from the Miocene period). 

 Species of Mya, Tellina and Saxicava are very characteristic of the 

 Saxicava-Sand formation (Post-Glacial) of the Province of Quebec. 



FIG. 214. 



Mya truncata. 

 Post-Glacial. 



FIG. 215. 



Tellina groenlandica. 

 Post-Glacial. 



FIG. 216. 



Saxicava rurjosa. 

 Post-Glacial. 



CLASS II. SCAPHOPODA. 



This class comprises but one family, that of the Dentalidce, in 

 which the head is rudimentary (but furnished with a lingual ribbon 

 or radula) and the foot in the form of a hollow, conical organ, adapted 

 for boring whence the name of the class, from gxa^oc, a spade. The 

 typical genus Dentalium, in which the shell is in the form of a 

 hollow cone, straight or curved, and open at both ends, dates from 

 the Silurian or Devonian periods, but is without representatives in 

 our strata. 



CLASS III. PTEKOPODA. 



The Pteropods form a small series of pelagic, free-swimming mol- 

 lusks, frequenters of the open ocean. They possess a rudimentary 

 head, with a fin-like, natatory organ on each side. Some are naked 

 types, others secrete a light shell of variable form. They are com- 

 monly classed in two orders : 1. Thecosomata, with, typically, a thin 

 external shell ; and 2. Gymnosomata, including the naked forms, 

 with more or less distinct head. The modern genera Hyalaea and 

 Limacina (both dating from the Miocene period) are types of the 

 first Order ; and the recent genus Clio is the principal representa- 

 sive of the second Order. 



