THENTON LIMESTONE. 155 



The three following species are of forms which have heretofore been referred to the genera 

 Cypricardia, Cardicm, &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. The 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 to it are of Silurian, 

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

 and the present, the genus has been absent 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. Shells often very gibbous, 



M. DE Verneuil has suggested to the Author, that several of these will fall under the 

 genus Edmondia of de Koninck. Not being able fully to ascertain the characters on which 

 this genus is founded, I place them with some hesitation under that name. It may probably 

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



197. 1. EDMONDIA VENTRICOS A (n.5j9.). 



Pi.. XXXV. Figs. 1 a, b, c, d, e,f. 



Rhomboidal, cuneate, 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, cuneate ; surface 

 with fine concentric stria;. 



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, 

 diminishing rapidly in thtit direction. The specimens usually found are casts, and suffer 

 various distortions from compression in different directions, giving, in some instances, a 

 widely different form to the shell. Anterior profile cordiform. 



Fig. I a. Right valve of a large and perfect specimea 



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



Fig. 1 e. Dorsal view of the same. 



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



Fig. 1 e. 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 sheila, suffering from pressure in the soft shaly rocks, become coated by a thin striated film of shale, marked 

 like the " slickensides," » 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 the shell, having also a striated surface. 

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

 hut it happens equally to other shells. 



20* 



