MOLLUSCS. 



275 



(•()\ t'l'i'd witli an ambei- or Vnowii I'pideiMiis, while in the brackish waters of wanner 

 countries occur some larger forms. The family under which these are assembled is 

 variously known as Cycladid^ or Cyeenid^, the latter name being prefei-able. In 

 all, the shell is nearly circular in outline, the ligament is external, the hinge is provided 

 with several teeth. Usually there are apparent indications of a pallial sinus, most 

 niai'ked in the American species. 



Cyrena is the typical genus and embraces over a liundicd nominal species, which 

 live in brackisli water in the warmer parts of the globe. They are frequently found 

 liuried in the mud of mangrove svvamjjs, where the tide rises and falls slightly, but 

 where the admixture of rain renders the water less dense than that of the ocean. 

 Cyrena carolinensis occurs in the rivers and swamps of some of our southern states. 

 In our northern states the family is rejiresented by the genera Sjyharium and Pit:i- 

 iliuiti, our fauna containing about fifty species of both genera. Spharium (known in 

 some of the older works as Cyclas) has the shell nearly equilatei'al, the hinge teeth 

 minute and rather weak, and two nearly sejjarate siphons. In Pisidium the part of 

 the shell in front of the umbones is larger than that behind, the teeth are stronger, 

 and the two siphons are united the whole length. The sjiecies abound in the still 

 water of some of our ponds, 

 and are very active. 



The C VPR1NID.E is a much 

 larger family than the last, 

 .and its members are iidiabi- 

 tants of salt watei-. Tin 

 shell is regular, oval, and 

 equivalve, and is covered 

 with a thick, strong epider- 

 mis. The hinge ligament is 

 usually external, and the 

 hinge is provided with from 

 one to three cardinal teeth, 

 and usually one posterior 

 lateral in each valve. The 

 margins of the mantle are 

 fringed, the pallial line sim- 

 ple, and the two sijihonal 

 tubes are short. 



Cyprina islandica is a 

 large boreal shell, common in sandy bottoms north of Cape (Jod, but is less frequently 

 met with south of that barrier. The hinge has three 

 unequal diverging cardinal teeth and one latci-al. 

 The shell is thick and heavy, and the color in the 

 young is very light brown, but old specimens ai-e 

 very dark. With age, the epidermis, near the um- 

 bones, usually disappears, and the shell itself is fre- 

 quently eroded. Large specimens measure four 

 Several species of Astarte are found on our northern 

 coasts, all of which can be recognized by the smooth or concentrically furrowed surface, 

 .and the two hinge teeth in each valve. The shell is covered with a strong epidermi."!. 



- ''.'//"• 



inches across. 



Fig. 295. — Vt/rln- 

 cardia novanylue. 



