106 INTRODUCTION. 



Terebratulites (part), Schloth. 



Strophomena, Raf., Blainville, and other Authors. 



LepTjENA, Dalman, and of the generaUty of Authors. 



Leptagonia, M'Coy. 



Orthis (part) of V. Buck. 



Producta (part), of Phillips and Beshayes. 



Animal unknown ; shell depressed, expanded, semicircular, sub-quadrate, transverse 

 or elongated ; external surface smooth, variously striated or costated. Hinge line straight, 

 generally as long as the width of the shell ; dental or ventral valve convex or concave ; 

 socket or dorsal valve following the curves of the other, the margin in some species being 

 suddenly and abruptly bent down when the shell is half grown ; area double, crenulated 

 at its inner edges, and more developed in the dental valve ; fissure in larger or ventral 

 valve partly covered by a deltidium, the extremity of the beak entire, or perforated by 

 a small circular aperture which became closed at a more advanced period of the animal's 

 life. In the socket valve, the fissure is either partially covered by a deltidium, or entirely 

 occupied by a projecting cardinal boss. In the interior of the dental valve, two widely- 

 diverging teeth articulate with corresponding sockets in the other valves ; the muscular 

 impressions are more or less distinctly margined by a semicircular ridge continued 

 from the base of the teeth, and curving on either side so as to produce a saucer-shaped 

 depression of variable dimensions. The cardinal muscles fill on either side the anterior 

 portion of this cavity ; the pedicle muscles leave no definite scars, unless the external 

 portions of the " cardinal muscular impressions " are due to them. The adductor lies 

 close to a slightly elevated mesial ridge. In the interior of the smaller valve the cardinal 

 process or boss is large and divided into two lobes, and not connate with the divergent 

 socket ridges; to this process were no doubt affixed the cardinal muscles,. From this 

 a slight mesial ridge runs down, and separates the two pairs of adductor scars, which are 

 frequently bordered by prominent ridges ; the vascular impressions consist of large primary 

 vessels which run at once direct to the margin, following a somewhat radiate arrangement, 

 or else the lateral trunks are greatly enlarged and reflected to surround the ovarian spaces. 



Ohs. Much has been written on the shells above described, and many have been the 

 opinions entertained, and sub-divisions proposed, for Rafinesque's and Dalman's genera 

 Strophomena and Leptana) Some authors have adopted one name in preference to the 

 other for the same species, while a few would preserve both, but all are not agreed in what 

 manner the division should he made. It will not be necessary to enter into the dis- 



1 Petre/acta suecana, in Kongl. Vet. Acad. Handl., 1828. 



1. Leptcena, " sub-equivalve, flattened, with the margins compressed and bent, hinge line 

 straight, very wide ; foramen 0. ; one valve with two bhint teeth. First example, L. ruyosa. 

 His. Second, L. depressa, Sow. Dalman speaks of Leptaena, as equal to part of Sowerby's 

 genus Productus, which he abolishes. In the supplement to the Mineral Conchology, Mr. 

 James De Carle Sowerby adopted Leptcena as a substitute for Productus ! 



