SIPHONIDA. 507 



it carries an internal ligament supported upon a prominent 

 cartilage -process (figs. 373, 374). The My as 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. 376) the shell is inequivalve, the left 

 valve the smallest, and with a prominent cartilage- process ; 

 but the shell is gibbous, and does not gape at its ends, whilst 

 the pallial sinus is small. Numerous fossil species are known, 

 commencing in the Lower Oolites. 



Fig. 375. Saxicava rugosa, left valve. Fig. 376. Corbula pisum, viewed from 



Post-Pliocene and Recent. the left and right sides. Eocene. 



Necera, commencing in the Jurassic period, is allied to 

 Corlula, but the shell is nearly equivalve, the right valve 

 being slightly the smallest, and the shell is produced and 

 open posteriorly. 



The genus Thetis is a small one, including thin, trans- 

 lucent, sub-orbicular shells, with an external ligament. A 

 few species of the genus are known, commencing with the 

 Lower Cretaceous rocks. Poromya, of the Eocene, appears to 

 be related to Thetis. 



Panopcea resembles Mya in having a thick oblong shell, 

 gaping at each end ; but the shell is equivalve, and the liga- 

 ment is external. Very numerous fossil species of this genus 

 are known, commencing in the Lower Oolites. Glycimeris, 

 with Tertiary and Eecent species, is nearly allied to Panopcea, 

 the only living forms being characteristic of the Arctic seas. 



Saxicava, as its name implies, includes shells which form 

 burrows in rocks. The adult shell (fig. 375) is edentulous, 

 equivalve, and oblong, gaping at the ends, and furnished with 

 an external ligament. The genus seems to commence in the 

 Eocene Tertiary, and has continued to the present day. 



