15- 



PLICATULA. 



Laju. — Syst. des anini. sans vert. p. 132. 



TESTA irregularis insequivalvis^ inauriculata^ basi 

 atteniiata; margine supero rotundato^ subplicato ; 

 umhonibiis insequalibus; areis externis iiullis. — 

 Cardo deiitibus duobus validis in utraque valva. 

 Fovea intermedia ligamentum omnino internum 

 recipiens. 



The above Generic character appears evidently to have 

 been intended by Lamarck, to distinguish this Genus from 

 Pecten and Spondjjlus^ to both of which it has some affi- 

 nity, but particularly to the latter, in which the species 

 of it have been placed by Linne^ Gmelm^ and others, whose 

 Spondj/lus plicatus may be considered as the type of the 

 Genus. 



The two valves in Plicatula are not alike, one being- 

 rather convex and attached by its outside, generally near 

 the umbo, to submarine bodies ; the other being flatter in 

 most of the species, and somewhat so in all. The general 

 form of the shell is like an oyster, without the auricles, 

 which distinguish Pecten and some of the other Genera 

 included in the Linnean Ostrea; the base or umbo is at- 

 tenuated, and the opposite margin rounded, and generally 

 more or less plaited ; the umhones are unequal, that is, 

 the umbo of the attached valve is more produced than the 

 other ; their points are not separated by any external area 

 or disk, as in Spondylus. Two distinct, generally perpen- 

 dicularly grooved teeth are seen in each valve, these have 

 their points turned backwards, and they are so locked into 

 each other that the two valves cannot be separated without 

 breaking them, nor can they be opened wide without using 

 extraordinary violence ; in the center of the hinge between 



