274 LOWER INVERTEBRATES. 



south of Cape Cod. Serripes is noticeable from the fact that the cardinal hinge teeth 

 have disappeared ; the species S. gronlandica occurs in the polar seas, extending south 

 on our coasts to Massachusetts Bay. Ilemicardiurn is a tropical genus, in which the 

 posterior part of the shell is separated from the anterior by an abrupt angle. The 

 cardinal teeth are distinct. 



Fig. ^91. — Carrfium echinatnm , cockle, with the foot extended. 



The LuciNiD^ embrace forms with a circular outline, one or two cai-dinal hinge 

 teeth, and the laterals one or obsolete. The hinge ligament is external or partially 

 internal. The mantle is open in front, but behind is united into one or two short 

 siphonal tubes. The foot is long, cylindrical, or worm-shaped. Lucina is represented 

 on our coast by two species, one of wljich, X. deutata (sometimes included in a sub- 

 genus Cyclas) is noticeable for the peculiar ornamentation of the 

 valves. The shell is j)ure white, and exhibits the regular lines of 

 growtli, but over these is laid a second series of lines in a manner 

 difficult to describe in a few words, but readily understood from 

 tlie figure. One or two other genera {Stric/illa, Choristodon) be- 

 longing to the Tellinidas are ornamented in a similar manner. 

 Fig. m&^Lucina den- In Lucina filosa, which is not ^•ery common, these striations 



'"'"■ are absent, and the shell is roughened by a number of concentric 



lamellar ridges witli smaller, thread-like ridges between. 



In Crypttdon the hinge teeth are wanting. Ungulina., a genus of the eastern seas, 

 is noticeable from the fact that it excavates winding galleries in the coral on the reefs 

 where it is found. Kellia jikmidata is found on the northern coast of America, under 

 stones, at low-water mark, and other species in various parts of the world live above 

 the reach of ordinary tides, or burrow in sandstone. They creep about freely, and 

 anchor themselves by a byssus at pleasure. One species of Montacuta occurring in 

 Great Britain {M. substriata) is to be mentioned because it has never been found 

 except attached to the spines of the sea-urcliin, Spatangiis pnrpureus. It cannot be 

 regarded as a true parasite, for not only has it no organs by which to feed upon the 

 urchin, but it is never attached to any of the soft parts of its host. It is rather to be 

 regarded as a mess-mate or commensal, obtaining its food from the same source as 

 does the urchin. 



In the fresh waters the world over occurs a group of usually small bivalve shells, 



