CYRENA. 



Shell subtriangular, with much rounded angles, ven» 

 tricose, equivalve, inequilateral. Cardinal teetn in each 

 valve, three ; lateral teeth, two ; in one valve the posterior 

 one is placed near the hinge teeth, the anterior one being 

 farther off and before the ligament ; in the other valve 

 there is a deep groove placed between two teeth, one of 

 which is large, and the other nearly obsolete. Ligament 

 external, on the larger side. Muscular impressions two, 

 lateral, distant. 



In the species which we have represented a circum- 

 stance of rather singular nature is observable ; (also to be 

 seen in many specimens of the fossil species of the Isle of 

 Wight,) if the outside of the shell between the umbones 

 where it is much eroded be examined, the remains of the 

 older portions of the hinge teeth will be seen standing out 

 in relief, showing that they have resisted the action of the 

 eroding matter better than the shell itself ; which we be- 

 lieve to have arisen merely from the different position of the 

 molecules of which they are formed. 



Several fossil species of CyrensB are found in great 

 abundance in the stratum, commonly called " the upper 

 marine formation" in the Isle of Wight, but which we have 

 elsewhere shown* to consist of a mixture of marine and 

 fresh water shells, in which the latter predominate. They 

 occur also in a somewhat similar stratum at Woolwich and 

 in some other places in the vicinity of London, as well as 

 at Hordwell ; and near Paris where this kind of mixed 

 stratum has been long recognized. 



Having thus published a species which we believe ta 

 be hitherto undescribed, we are compelled to name and 

 define it, though this is a task we did not intend to under- 

 take when we commenced this work. 



Cyrena Sumatrensis. Testa ovalis, gibbosa, crassa : 

 dentibus duobus cardinalibus majoribus angulatis, sub- 

 bifidis. 



Brought from Sumatra by Mr. Griffiths : the specimens 

 we have seen are white within, but we are informed that 

 the inside is sometimes of a beautiful purple colour : the 

 angular, nearly bifid, two larger cardinal teeth distinguish 

 it from all the other species we have seen as well as from 

 those described by Lamarck. 



Several of tbe fossil species will be found represented 

 in Sowerby's Mineral Conchology, under the Generic name 

 Cyclas. 



* Sec Annals of Philosophy, Au^. 182L 



