BRACHIOPODA. 



459 



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 



Fig. 313. Discina Circe. 

 Silurian. 



Fig. 312. Crania Ignabergensis. Cretaceous. 



valve, by means of a slit in its hinder portion or a circular 

 foramen excavated in its substance. Arms fleshy. Valves 

 unarticulated. The Discinidce range from 

 the Silurian period to the present day. 

 The four most important genera are Dis- 

 cina, Trematis, Siphonotreta, and Acrotreta. 



In Discina (figs. 313, 314) the shell is 

 generally circular or orbicular in shape, 

 and is horny in texture. The upper valve 

 is limpet-shaped, smooth or concentrically 

 striated ; the ventral valve is flat or partly 

 convex, perforated by a longitudinal slit, which is placed 

 in the middle of an oval depressed disc. The valves are 

 not articulated to one another, but are kept together by 

 muscular action alone. The species of the 

 genus Discina (including Orbiculoided) range 

 from the Silurian rocks to the present day, 

 seven species being now living. 



In the genus Trematis (fig. 316, A) 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 marginal slit for the passage of a peduncle of at- 

 tachment. The surface is radiately striated, and orna- 



Fig. 314. Discina 

 Pelopea. Silurian. 



