PTEROPODA. Ill 



but the shell is not perforated ; two curious bent pro- 

 cesses jut out from under the shorter or lower beak. 



II. Terebratula cajQut-serpeniis, or Serpent's head, from 



a certain flatness and tapering form ; the valves are 

 striated ; inside, the long ciliated arms are supported 

 pn an arched and looped appendage attached to the 

 lower valve ; the byssal cord passes through a perfo- 

 ration in the beak of the larger. 

 T. cranium has the shell smooth. 



III. Megathyris cistellula : small, broad, with a straight 

 dorsal edge, a triangular opening through the disc on 

 the larger valve. 



CRANIAD^. 



Crania anomala was first found adhering to stones from 

 deep water in Zetland; it has since been taken in other 

 places : it is like the Terehratula in some respects, but the 

 lower valve grows on the stone to which it has been fixed, 

 and the upper is something like a square limpet. The pecu- 

 liar arrangement of the four muscular impressions gives, in 

 some species, the appearance of a mask. 



PTEKOPODA. 



The Hyal^a trispinosa and the Spiralis Flemingii, 



