184 



arch, comprising the whole of the lower margin of the shell, with two remote 

 projecting lateral teeth. Whether there exists in the other, as I believe, 

 a ridge corresponding with the groove in this valve, or whether it was 

 opposed by a corresponding groove, must remain to be ascertained by 

 the examination of some more illustrative specimen. 



CXIV. Trigondlites. A slightly-rounded, trigonal, thick shell, gaping 

 on each side. The anterior margin nearly on a straight line ; the pos- 

 terior in a gently waving, and the upper side in nearly a circular direc- 

 tion. The outer surface of each valve thickly pierced by foramina, 

 which, passing nearly through its substance, gives it the cancellous ap- 

 pearance of bone : the inner surface smooth, but marked with striae, 

 concentric with the upper margin. The hinge completely linear, with- 

 out teeth ; there being only an appropriate surface, on the anterior mar- 

 gin of each valve, for the attachment of the cartilage externally. No 

 appearance of muscular attachment. 



This shell, which I have presumed thus to name and to dispose of, is ex- 

 ceedingly anomalous in every respect : it is of an extraordinary degree of 

 thickness, being, in some specimens, full three-eighths of an inch thick; 

 and, even in young specimens, full a quarter of an inch thick. A fbs- 

 sula, or at least a greater degree of concavity, is observable just beneath 

 the beak, and which is the only part which bears the least appearance of 

 the animal having been attached to it. Possessing both valves, I have care- 

 fully sought to discover the state in which they would be, if united ; but 

 can only ascertain, that if brought into contact at the angular points, by 

 approximating first the anterior sides of the valves, the shell gapes on 

 each side, the rotundity of the shell occasioning a separation as the shell 

 closes, of even the anterior margins, which are formed nearly on a 

 straight line ; and which, when the valves are widely open, lay in a line 

 with each other, as at Plate XII L Fig. 10 and 11. If the superior 

 margins, which alone can be thus brought into contact, are placed toge- 

 ther, the shell is very widely separated at the beaks. 



This shell was first noticed by Scheuchzer as Concha fo&silis tdliiwdes. 



