SIPHONIDA. 499 



with numerous Tertiary species, has two cardinal teeth and 

 an internal ligament. Lastly, the genera Montacuta, Lepton, 

 and Galeomma, are all represented by fossil forms in the 

 Pliocene Tertiary. It is probable that the genus Axinus 

 (not the same as Schizodus), with various Tertiary species, 

 should be referred here ; and we may perhaps provisionally 

 include in this family the singular genus Solemya, though its 

 shell is elongated and cylindrical, gaping at both ends, and 

 its hinge is edentulous. This genus has been variously 

 placed, and is sometimes regarded as the type of a distinct 

 family (Stoliczka). It begins as early as the Devonian, has 

 several Carboniferous forms, and is represented by a few 

 species at the present day. 



FAM. 12. CYCLADID^E. Shell sub-orbicular, closed; hinge 

 with cardinal and lateral teeth ; ligament external. Mantle 

 open in front ; a single siphon, or two more or less united. 

 Foot large, tongue-shaped. The genera Cyclas and Cyrena 

 are the two most important members of this family, and 

 both are inhabitants of fresh water ; though the latter not 

 uncommonly frequents brackish water, and one species of 

 the former has been described as marine. 



In the Cyclades the shell is thin, and there are two hinge- 

 teeth in one valve and one in the other. In Cyclas itself 

 the shell is nearly equi- 

 lateral, but in the sub- 

 genus Pisidium it is in- 

 equilateral, with the an- 

 terior side the longest. 

 In Cyrena (fig. 365) the 



Shell is thick, and there Vlg. W.Cyrena avtigva. Eocene. 



are three hinge -teeth in 



each valve. Both Cyclas and Cyrena seem to have come 

 into existence at the commencement of the Cretaceous period 

 (Wealden), and they are abundantly distributed through the 

 Tertiary rocks. 



FAM. 13. CYPRINID^E. Shell equivalve, closed; ligament 

 external ; cardinal teeth 1-3 in each valve, and usually a 

 posterior tooth. Mantle-lobes united behind by a curtain 

 pierced with two siphonal orifices. Foot thick and tongue- 



