212 MOLLUSCA. 



single genus Crania, which commenced in the Silurian period, 

 and has continued to exist without interruption to the present 

 day. The shell may be smooth, or striated with radiating 

 ribs, sometimes with spines or foliaceous expansions. The 

 ventral valve is the shallowest, and the shell is usually attached 

 by a portion of its substance. The dorsal valve is more or 

 less conical, and the valves are simply kept in apposition by 

 muscular action. 



FAM. IX. DISCINID^E. Animal attached by means of a 

 muscular peduncle passing through the ventral or lower valve, 

 by means of a slit in its hinder portion or a circular foramen 

 excavated in its substance. Arms fleshy. Valves unarticulated. 

 The Discinidfr range from the Silurian period to the present 

 day. The three most important genera are Discina, Tr emails, 

 and Siphonotreta. 



In Discina (fig. 162) the shell is generally circular or orbicu- 

 lar in shape. The upper valve is limpet-shaped, smooth or 



Fig. 162. Disci>ui Circe, Silurian. Fig. 163. Disciiia l j elopea, Silurian. 



concentrically striated ; the ventral valve is flat or partly con- 

 vex, perforated by a longitudinal slit, which is placed in the 

 middle of an oval depressed disc. The valves are not articu- 

 lated to one another, but are kept together by muscular action 

 alone. The species of the genus Discina range from the 

 Silurian Rocks to the present day, seven species being now 

 living. 



In the genus Trematis both valves are more or less convex, 

 and the general shape of the shell is more or less oval or sub- 

 orbicular. The ventral valve is furnished 

 with a slit for the passage of a peduncle 

 of attachment. This genus seems to be 

 exclusively Silurian. 



In the genus Siphonotreta (fig. 164) 

 the shell is oval, inequivalve, with un- 

 articulated valves. The beak of the 

 ventral valve is perforated by a foramen 

 which opens on its back, and communicates with the interior 

 by a cylindrical tube. The surface of the shell is covered with 



