238 



MOLLUSCA. 



tile. Foot very small. The more important genera of this 

 family are Mya, Corbula, Thetis, Panopcea, and Saxicava. 



In the Gapers (Mya}, 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 (fig. 198). The Myas live buried verti- 

 cally 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 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, commenc- 

 ing in the Lower Oolites. 



The genus Thetis is a small one, including thin, translucent, 

 sub-orbicular shells, with an ex- 

 ternal ligament. A few species 

 of the genus are known, com- 

 mencing with the Lower Cre- 

 taceous Rocks. 



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. 



Saxicava, as its name implies, 

 includes shells which form bur- 

 rows in rocks. The adult shell 

 (fig. 199) is edentulous, equi- 

 valve, and oblong, gaping at the 

 ends, and furnished with an ex- 

 ternal ligament. The genus seems 

 to commence in the Eocene 

 Tertiary, and has continued to 

 the present day. 



FAM. 19. ANATINID.E. Shell 

 often inequivalve, with an external 

 ligament. Mantle-lobes more or 

 less united. Siphons long, more 

 or less united. Foot small. The 

 more abundant and important 

 fossil genera of this family are 

 Anatina, Pholadomya, and Myacites. 



Fig. 199. Saxicava rugosa, 1 

 valve. Post-Pliocene and Recent. 



Fig. 200. Anatina sfxitulata. 

 Kimmeridge Clay (Upper Oolites* 



