MYACEA. 



743 



Fig. 628. — Corbida pisum, viewed from 

 the left and right sides. Eocene. 



gibbous, and does not gape at its ends, whilst the pallial sinus is 

 small. The recent species of Corbula live partly in the sea, and 

 partly in estuaries or at the mouths of rivers. There are numer- 

 ous fossil forms of the genus, espe- 

 cially in the Cretaceous and Ter- 

 tiary rocks ; but the oldest forms 

 appear in the Trias. 



The genus Necera is allied to Cor- 

 bula , but the shell is nearly equi- 

 valve, the right valve being slightly 

 the smallest, and the shell is pro- 

 duced and open posteriorly. The 



earliest types of Near a appear in the Upper Jurassic rocks, and 

 the genus still survives. 



Family 3. Glycimerice. — In this family the mantle-lobes are 

 united, and there are long siphons, which are united wholly or in 

 great part. The shell is equivalve or nearly so, gaping at both ends, 

 and covered with a thick epidermis, which is prolonged over the 

 siphons. The hinge has one or two weak cardinal teeth, or is tooth- 

 less. The ligament is external, and the pallial sinus varies in depth. 

 The Bivalves included in this family are all marine, and are either 

 free or burrow in mud. 



In Glycimeris ( = Panoficed) the shell (fig. 629, a) is thick, oblong, 

 inequilateral, and gaping at both ends. A single cardinal tooth is 





Fig. 629. — A, Interior of the right valve of Glycimeris (Panoftaa) Americana, from the 

 Miocene rocks of North America, one-third of the natural size ; b, Interior of the left valve of 

 Cyrtodaria (Glycimeris) siliqua, Arctic seas, two-thirds of the natural size. (After Woodward.) 



present in each valve ; the pallial line is continuous, and the pallial 

 sinus is very deep. Several Recent species of Glycimeris are known, 

 and there are also various Tertiary forms, the earliest types of the 

 genus seeming to occur in the Cretaceous rocks. Allied to the pre- 

 ceding is the genus Cyrtodaria ( = the Glycimeris of many authors). 

 in which the shell agrees generally with that of Glycimeris, but the 

 pallial line is interrupted, and the pallial sinus is very slight (fig. 

 629, b). 



In the genus Saxicava (fig. 630) the animal burrows in rocks, or 

 fixes itself in crevices by means of a byssus. The adult shell is 



