104 MOLLUSCA FROM THE GREAT OOLITE. 



upon the casts, a feature similar to that in Ceromya ; the muscular impressions are very 

 faintly marked, as is likewise the pallial impression, the flexure of which appears to be 

 short. This genus, having been reunited to Ceromya by M. d'Orbigny, and M. Buvignier 

 having figured and described the hinge of a Gresslya, named by him Ceromya Deshayesi, 

 in a very complete manner, it has become necessary to institute a close comparison between 

 the two generic forms, and to weigh carefully their affinities and differences 



Istly. Form. Ceromya is usually larger thau Gresslya, and always more ventricose, 

 the urabones are more prominent, those of Ceromya approaching to the form of Isocardia ; 

 Gresslya, with its acute umbones and more compressed figure, approaches to that of 

 Cardinia : Gresslya is also very constantly slightly insequivalve, the right valve exceeding 

 the other in height ; in Ceromya they are equal, and any irregularity of form which may 

 sometimes occur to give the semblance of inequality in the valves is altogether accidental, 

 and depends, apparently, upon the portion of the shell during its growth. 



2dly. Character of the Surface. The sculptured surface of Ceromya is quite unlike 

 that of any other of the Myadts, the longitudinal grooves being more or less visible upon 

 the casts, but the casts of Gresslya are smooth, and the granulated surface of the test is 

 altogether different from that of Ceromya. 



3dly. Hinge Characters. In Ceromya, as in Gresslya, the casts of the right valve 

 exhibit a groove posterior to the umbones which has been impressed by a corresponding 

 prominence or internal rib in that valve ; in Ceromya, however, this groove is likewise 

 visible upon the exterior of the test, but not in Gresslya ; the internal hinge larninre are 

 precisely alike in both genera ; but this is a feature which in the fossil Myadce has but 

 little value in generic affinity or distinction. Whatever value the Palaeontologist may be 

 disposed to attach to the foregoing distinctions when viewed singly, it must, we think, be 

 admitted that in the aggregate they are of considerable importance, and it is necessary to 

 neglect none of them in forming a fair estimate of the two forms. 



Gresslya was eminently gregareous, Ceromya not so, and for the most part it occurs 

 much more sparingly ; both lived in the same beds ; the valves of Ceromya are frequently 

 disunited, in Gresslya they are invariably in contact. 



GRESSLYA CARDIT^EFORMIS. . 



Testa ovalo-depressd; umbonibusprominentibussubplanis, latere antico produclo rotundato, 

 basi curvatd, latere postico abrupte declivi, lineis incrcmenti paucis, irregularibus. 



Shell ovate, depressed ; umbones prominent, rather compressed ; anterior side produced 

 and rounded ; base curved ; posterior side sloping abruptly ; lines of growth few and distant. 



This species possesses a general resemblance to Gresslya Saussuri, the Venus Saussuri 

 of Brongniart and Goldfuss, but our shell has much less convexity ; in both species the 

 outline has a considerable resemblance to that of a Venus, but an examination of the hinge 

 border has proved that it is edentulous. 



