PECTEN, 



TESTA insequivalvis, subsequilateralis, pleriim- 

 que radiatim sulcata, valvis auritis, auriciilis 

 inaequalibiis ; sinu byssi in altera valva; impres- 

 sione miisculari magna sublaterali ; impressione 

 7nusculari pallii absque sinu; cardine lineari ; 

 ligamemto tripartite, partibus duabus laterali- 

 bus elongatis, lineam cardinalem, rectani, se- 

 quentibus, tertia parte triangulari, erassa, in 

 fossula interna, ceatrali, cardinis posita. 



The first observ^ation that presents itself to our mind in 

 commencing* om' account of this extensive and beautiful 

 genus is the difficulty of deciding upon the propriety or 

 inipropriety of dividing it into several separate genera, 

 or effecting nearly the same useful and necessary object, 

 by gn>uping the species under several divisions of one 

 genus; we have chosen the latter proceeding, because we 

 think it easier for the learner, as directing his attention in 

 the first place to the general characters of a large group, 

 and then pointing out the peculiarities of the several 

 smaller parcels which compose it, and thus leading by 

 degrees to the specific characters of each individualo 



The Pectines are all more or less inequi valve, even 

 those that are nearly lenticular, having one valve rather 

 smaller than the other: sometimes they are nearly equi- 

 lateral, but generally they are inequilateral ; for the most 

 part they are covered with ribs, or grooves radiating from 

 tlie umbones to the margins; on either side of the umbo 

 m each valve may be always observed an irregularly tri- 

 angular appendage, commonly called an ear; these ears 

 are unequal in size : immediately below om Qf Jhejn in 



