BIVALVIA. 171 



1. ERYCINELLA OVALIS ? Conrad. Tab. XV, fig. 10 a -f. 



ERYCINELLA OVALIS 1 Conrad. FOBS, of the Miocene Formation of the United States, p. 74, 



pi. xlii, fig. 5, 1845. 

 GOODALLIA? PYGM^EA. S. Wood. Catalogue, 1840. 



? CRENATTJLA. Id. 



Spec. Char. Testa minutd, obligud, ovato-trigonuld, incequilaterali, tumidd, crassd, 

 concentrice striatd; postice rotundatd, antice rectiusculd ; cardine bidentato ; margins 

 ventrali cremilato. 



Shell minute, oblique, ovately triangular, inequilateral, tumid, thick and strong, 

 concentrically striated ; posterior side rounded, anterior nearly straight ; hinge with 

 two teeth, ventral margin crenulated. 



Diameter, -^h of an inch. 



Locality. Cor. Crag, Sutton, and Gedgrave. 



This little shell is particularly abundant in the Coralline Crag at Sutton ; and from 

 its prominent dental apparatus and crenulated margin, the two valves may be often 

 obtained in their natural position, giving thus an opportunity of observing the entire 

 margins of the shell in juxtaposition, without the slightest appearance of any fulcrum 

 or depression into which the ligament could have been inserted, so as to have become 

 visible externally when the valves are closed, the two pieces locking so close together 

 as to be with difficulty separated. The shell is thick and strong, with a prominent 

 obtuse umbo, and the whole exterior, when in perfect condition, is covered with 

 concentric ridges or rounded striae. The hinge of the right valve consists of 

 two large teeth diverging from the umbo, having a triangular space between them, 

 and a small obscure lateral tooth on both sides; in the left valve there are two 

 large cardinal teeth also diverging, which lock into depressions on the outside of 

 the teeth of the right valve ; besides these two cardinal teeth in each there are two 

 small denticles in the left, between which and the larger teeth are spaces for the 

 reception of the two large teeth of the right valve, thus leaving in the centre a vacant 

 space immediately beneath the umbo, where it is presumed the ligament was inserted ; 

 and in this valve also are two lateral teeth, the one at the extremity of the slope of the 

 anterior margin is prominent, fitting into a corresponding depression in the anterior 

 margin of the right valve, while on the other side it is the reverse, corresponding in 

 this character with the lateral teeth of the small species of Astarte, to which it is 

 evidently nearly related. The edge of the margin is deeply denticulated in the adult 

 shell, but in the immature state it is perfectly smooth ; the adductor muscles are 

 deeply impressed, large, and of an ovate form, situated near the extremity of the 

 lateral teeth ; they are connected by a simple mantlemark, without the least inflection : 

 in the young state the teeth of the hinge are comparatively larger than when full 

 grown, and the margin is then without crenulations; from these, with some other minor 

 differences, it was thought to be a distinct species, but a better examination and more 

 specimens give reason to believe it to be only the immature state. 



