BIVALVIA. 17 



Umbones anteriores parvi depressi. Impressiones musculares duee, in utraque valvis 

 anticis parvis, poslicis magnis ellipticis. 



Fades externa sulco longitudinali elongato, sub-marffinem cardinalem sitam. 



Shell nearly equivalve, inequilateral, both extremities winged, anterior wing short, 

 posterior elongated, its extremity marginated. 



Hinge margin straight, more or less oblique, ligamental area internal, and nearly 

 parallel with the external margin. Hinge with numerous very small parallel teeth placed 

 beneath the umbones near to the anterior extremity of the shell, and one or two posterior 

 or internal costse, which are elongated and extend posteriorly nearly parallel with the hinge 

 margin. Umbones anterior, small and depressed. Muscular impressions two in each 

 valve, of which the anterior or byssal are very small ; the posterior expanded, elliptical, 

 and not strongly marked. External surface either ornamented or plain, having an 

 elongated longitudinal groove extending posteriorly to the umbo, and parallel with the 

 hinge margin in each valve. 



This remarkable Oolitic form replaces and represents the genus Pterinea of the 

 Palaeozoic formations ; its affinities to that genus are so evident, that it is necessary to 

 inquire whether Pteroperna should be arranged as a sub-genus of Pterinea only, or is 

 entitled to rank as a distinct genus. The principal distinguishing features are internal. 

 In Pterinea the posterior elongated accessory ribs or teeth proceed obliquely downwards 

 towards the inferior and posterior extremity of the valves, as far as the border of the large 

 posterior muscular impression, at the anterior side of which they terminate abruptly; the 

 muscular impression is angular, and extends upwards nearly to the hinge margin 

 posteriorly. In Pteroperna, on the other hand, the posterior costae extend along the 

 inner surface of the hinge margin, almost parallel with it, and are consequently placed 

 upon the hinge plate above the muscular impression, which is rounded or elliptical, and 

 placed lower or more nearly to the middle of the posterior surface. As minor points of 

 distinction it may be mentioned, that iu Pterinea the anterior teeth vary in number from 

 two to four only; but in our typical shell, Pteroperna costatula, they are not less than 

 sixteen, and are so minute that they scarcely occupy a greater longitudinal space than 

 those of Pterinea. Externally our genus possesses a characteristic feature very convenient 

 for the Palaeontologist, who is rarely able to refer to the hinge, and which readily serves 

 to distinguish it both from Pterinea and Avicula; we allude to the elongated posterior 

 groove, which is always visible upon the surface, and of which the other two forms are 

 destitute. Regarding, therefore, the position of the internal ligamental groove and 

 accessory costae, together with the form and position of the posterior muscular impression 

 as indicating a corresponding difference in the structure of the animal, when compared 

 with those parts of Pterinea, we consider ourselves justified in considering the Oolitic 

 form as a genus distinct from but nearly allied to Pterinea. 



Pteroperna also presents considerable analogy to the recent genus, Malleus. 



