274 



L O WER IN VER TEBR A TES. 



south of Cajie Cod. Serripes is iiotict-uble from tlie f;ict that thu cardinal liinge teeth 

 have disajjpeared ; the species «S'. (/ronlandica occui's in the pohir seas, extending south 

 on our coasts to Massachusetts Bay. Hemicurdinm is a tropical genus, in which tlie 

 posterior j)art of the sliell is separated from the anterior by an ahrujit angle. The 

 cardinal teeth are distinct. 



Fig. 292. — lALCina den- 

 tatfi. 



The LuciNiD.E embi-ace funiis with a circular outline, one or two cardinal hinge 

 teeth, and the laterals one or nlisiilcte. The hinge ligament is external or ]iartially 

 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. Tjuchio is represented 

 on our coast by two species, one of wl)ich, X. deiituta (sometimes included in a sub- 

 genus Ci/clas) is noticeable foi- the peculiar ornamentation of the 

 vaUes. The shell is ])ure white, and exhibits the regular lines of 

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

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

 tlie figure. One or tyvo other genera {iStn'r/illd, Choristodon) be- 

 longing to the Tellinida; are ornamented in a similar manner. 



In Lvcind ^filosd, which is not very common, these striations 

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

 lamellar ridges with smaller, thread-like ridges between. 



In Cryptodoii the hinge teeth are wanting. Unt/idina, 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. Kellin planidata is found on the northern coast of America, under 

 stones, at low-water mark, and other sjiecies in various ]iarts of the world live above 

 the reach of ordinary tides, or biuTow in sandstonv. 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 s])ines of the .sea-urchin, Sjmtam/iis pifrp2irens. It cannot be 

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

 nrchin, 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 urcliin. 



In the fresh waters the world over occurs a grou}i of usually small bivalve shells, 



