TBENTON LIMESTONE. li}.} 



The three following species are of forms w liiclx have heretofore boon referred to tiie genera 

 Cypricahdia, Cardium, &c.; but I am satisfied that this is an improper reference, and that 

 the shells have not the characters of these genera. They approach more nearly to Saxicava 

 and Venerirupis among recent shells, but they differ from any of these. Tiie reference of 

 Silurian fossils to Cypricardia should be regarded with distrust, since species of that genus 

 are so rare among recent shells, and all the fossil species yet referred lo it are of Silurian, 

 Devonian or Carboniferous age ; while thuing the long interval between the latter period 

 and the present, the genus has been al)sent from the fauna of the globe. 



The most obvious characters of the following species will be found in the rigid straight 

 hinge line, like Cucullea, with the umbones placed near to, or over, the anterior extremity, 

 which, in most instances, is abruptly rounded. Siiells often very gibbous. 



M. DE Verneuil has suggested to the Author, tiiat several of these will fall imder the 

 genus Edmondia of de Koninck. Not being aljle fully to ascertain tlic cliaracters on which 

 this genus is founded, I plac(; them with some hesitation nader that luuiie. It ui;iy probably 

 be found hereafter that they will require a separation from this genus. 



197. 1. EDMONDIA VENTRICOSA («. s;;.). 



Pl. XXXV. Figs. 1 a. i, ,-. ,/, e,f. 



Rhomboidal, ctmeate, ventricose above, with the umbones large and prominent, and 

 almost in a line with the anterior margin of the shell ; cardinal line short, from the posterior 

 extremity of which the shell slopes abruptly towards the ventral margin ; posterior dorsal 

 margin compressed ; anterior extremity obtuse ; posterior extremity acute, cuueate ; surface 

 with fine concentric striae. 



Nearly all the specimens seen are casts, with the surface markings more or less obscuie 

 or exfoliated. By placing the posterior extremity downwards, the shell is obtusely cuneate, 

 dimini.shing rapidly in that direction. The specimens usually found are casts, and suffer 

 various distortions from comj)ression in different directions, giving, in some instances, a 

 widely diflerent form to the shell. Anterior profile cordiform. 



Fig-. 1 a. Right valve of a large and perfect specimen. 



Fig. 1 b. Anterior extremity of the sumo, showing the ventricose character of the shell. 



Fig. 1 c. Dorsal view of the same. 



Fig. 1 d. Left valve of a smaller individual. 



Fig. 1 p. Left valve of a specimen which has been compressed vertically, and is covered by a fine striated 



lamina of shale.* 



Fig. 1 / Dorsal view of the same. 



* Many shells, siitlering from pi-essure in the soft shaly rocks, become coated by a thin striated film of sliale, marked 

 like the "slickonsidcs," as if the surrounding parts had been moved after partial solidification. In other instances, 

 there appears to be a collection of shaly matter in a concretionary form around tlie shell, having also a striated surface. 

 This aggregation of matter around the tube of the Orthoeeras, has been regarded as a portion of the animal petrified ; 

 liut it happens eijuallv tn other shells. 



20* 



