SIPHONIDA. 501 



In Astarte (fig. 366, c), the type of the second great group 

 of the Cyprinidce, the shell is thick and usually concentri- 

 cally furrowed, with a well-marked " lunule," and two car- 

 dinal teeth in each valve, the front tooth of the right valve 

 being large and thick. A very great number of fossil 

 species are known, but the genus appears to be wholly 

 Secondary, Tertiary, and Eecent. In the neighbourhood of 

 Astarte, however, we may place the Carboniferous genus 

 Astartella. 



Crassatdla (fig. 367) comprises thick, solid, ventricose 

 shells, attenuated posteriorly, and generally having a con- 



Fig. 367. Crassatella, ponderosa. Eocene Tertiary. 



centrically-furrowed surface. Unlike the preceding genus, 

 Crassatella has the ligament internal. There is a distinct 

 " lunule," and the cardinal teeth are two in one valve 

 and one in the other. The genus commences in the Cre- 

 taceous rocks, is abundant in the Tertiaries, and is well 

 represented at the present day. 



Pachyrisma is an extinct genus, in which the shell is also 

 very thick and ponderous in its structure. It has large sub- 

 spiral umbones, and is peculiar to the Great Oolite. 



Megalodon (fig. 368, A) is likewise extinct, and includes 

 massive shells, with sub-spiral beaks and an external liga- 

 ment. The genus is doubtfully represented in the Silurian 

 and Carboniferous rocks, and is characteristically Devonian. 



The Secondary genus Opis (fig. 368, B) has a heart-shaped 

 and keeled shell, with prominent incurved beaks, a distinct 

 lunule, and a single cardinal tooth in each valve. Still 

 another extinct genus is Cardinia (fig. 368, c), in which 

 the shell is trigonal or ovate, posteriorly attenuated, not 



