234 MOLLUSCA. 



The genus Conocardium comprises a number of Palaeozoic 

 shells, in which the anterior side is conical and gaping ; whilst 

 the posterior margin is truncated, and there is a longer or 

 shorter siphonal tube placed near the beaks. 



FAM. ii. LUCINID^E. Shell orbicular, free; cardinal teeth i 

 or 2; lateral teeth i - i, or obsolete. Mantle - lobes open 

 below, with one or two siphonal orifices behind; foot elon- 

 gated, cylindrical, or strap-shaped. The most important fossil 

 genera of the Lucinidcz are Lucina and Corbis. Nearly two 

 hundred species of the former have been described, commenc- 

 ing with the Devonian; and about eighty species of the former 

 are known, commencing with the Lias. 



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 Cydas and Cyrena 

 compose 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 Cydades the shell is thin, and there are two hinge- 

 teeth in one valve arid one in 

 the other. In Cydas itself 

 the shell is nearly equilateral, 

 but in the sub-genus Pisi- 

 dium, it is inequilateral, with 

 the anterior side the longest. 

 In Cyrena (fig. 193) the shell 

 is thick, and there are three 



Fig. 193. Cyrena autiqua. .Locene. . 



hinge -teeth in each valve. 



Both Cydas 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- 

 shaped. Of the genera of the Cyprinida, the more important 

 fossil forms belong to Cyprina, Astarte, Crassatella, Isocardia, 

 Cardita, and the extinct Megalodon, Anthracosia, Hippopodium, 

 and Pachyrisma. Taken as a whole, the Cyprinida have passed 

 their acme, and have begun to decline in numbers and im- 

 portance. 



Cyprina has a large, strong, oval shell, covered with a thick 

 epidermis. Numerous fossil species are known, commencing 

 in the Trias. 



