480 



LAMELLIBRANCHIATA. 



Tertiary and Secondary forms of small importance, character- 

 ised by the common feature that the ligament is lodged in 

 a single pit extending from the beak internally. Vulsella 

 (fig. 334, c) is distinguished by its nearly equivalve, earless 

 shell ; while the " Hammer-oysters " (Malleus) have very long 

 ears, and nearly central beaks. No fossil forms of the latter 

 are known with certainty, but several species of the former 

 have been detected in the Tertiary, and others have been 

 described from the Secondary rocks. 



A more important group of the Aviculidce is that repre- 

 sented by Perna and its allies, all of which have a straight 

 hinge-line, crossed by numerous transverse furrows for the 

 lodgment of the ligament. In Perna (or Melina) itself (fig. 

 339) the shell varies in form, but there is generally a long 

 posterior ear; there are numerous close -set cartilage-pits, 



Fig. 339. (A) Perna Mulleti, and a portion of its hinge-line (B). Lower Greensand. 



and the right valve has a byssal notch. The genus seems 

 to begin in the early part of the Secondary period, and is 

 especially well represented in the Cretaceous and Tertiary 

 deposits. A large and well-known species is the Perna 

 Mulleti of the Lower Greensand. Allied to the preceding 

 is the genus G-ervillia (fig. 340, A), in which the shell is 

 elongated and very oblique, with a broad and wing -like 



