MOLLUSCS. 



275 



covered with an amber or brown epidermis, while in the brackish waters of warmer 

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

 variously known as Cycladid^ or Cyebnid^, the latter name being preferable. 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 

 marked in the American species. 



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

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

 buried in the mud of. mangrove swamps, where the tide rises and falls slightly, but 

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

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

 In our northern states the family is represented by the genera Sphcermm and J'isi- 

 dium, our fauna containing about fifty species of both genera. Sphoerium (known in 

 some of the older works as Cyclas) has the shell nearly equilateral, the hinge teeth 

 minute and rather weak, and two nearly separate 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 species abound in the still 

 water of some of our ponds, 

 and are very active. 



The CypeinidjE is a much 

 larger family than the last, 

 and its members are inhabi- 

 tants of salt water. The 

 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 paUial line sim- 

 ple, and the two siphonal 

 tubes are short. 



Cyprina islandica is a 

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

 met with south of that barrier. The hinge has three 

 unequal diverging cardinal teeth and one lateral. 

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

 young is very light brown, but old specimens are 

 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 epidermis. 



Fig. 293. — Cyprina islandica. 



Fig. 294. — Astarte 

 wndata. 



inches across. 



Fig. WS. — Cuclo- 

 cardia novanglioi. 



