742 



LAMELLIBRANCHIATA. 



Mactra live buried in sand or mud. The fossil forms begin in the 

 Jurassic rocks, but the genus does not show any extensive develop- 

 ment till the Tertiary rocks are reached. 



In the genus Lutraria the shell is oblong and gaping at both 

 ends ; the pallial sinus is deep, and the internal ligament is sup- 

 ported by a prominent, spoon shaped 

 cartilage-plate. The earliest undoubted 

 fossil forms of this genus appear in the 

 Tertiary rocks, and about thirty living 

 species are known. 



Family 2. Myid^:. — In this family 

 the mantle is almost entirely closed, 

 but there exists in front an aperture for 

 the small triangular foot, while poste- 

 riorly are two long siphons, more or less 

 completely united with one another (fig. 

 555), and partly or wholly retractile. 

 The shell is inequivalve, thick, gaping 

 posteriorly, and not nacreous internally. 

 The internal ligament is supported upon 

 a spoon-shaped process developed from 

 the hinge-plate of the left valve. The 

 members of this family are marine or estuarine in habit, and the 

 principal genera are My a, Corbula, and JVecera. 



In the genus My a the shell is oblong, inequivalve, and gaping at 

 both ends. The left valve is the smallest, and it carries an internal 

 ligament supported upon a prominent cartilage-process (figs. 626, 



Fig. 625. — Mactra podolica. 

 ocene Tertiary 

 Zittel.) 



Austria. (After 



Fig. 626. — My a truncata. Post-Pliocene 

 and Recent. 



Fig. 627. — Portion of the hinge 

 of My a arenaria, showing the car- 

 tilage-process. 



627). The Myas live buried vertically in sand or mud. They are 

 not known to have existed before the period of the Middle Tertiary 

 (Miocene), and almost all the fossil species are in existence at the 

 present day. 



In Corbula (fig. 628) the shell is inequivalve, the left valve the 

 smallest, and with a prominent cartilage-process ; but the shell is 



