100 MOLLUSCA FROM THE GREAT OOLITE. 



Corimya is the Thracia of Leach, the latter author having the priority ; Tettina incerta, 

 Thurm., is au English Oolitic example. 



Mactromya has, we believe, justly been dismembered by D'Orbigny, the forms which 

 Agassiz regarded as typical having been separated from the Myadae to constitute the genus 

 Unicardium of the former author, and has been previously described in this Monograph. 

 Three remaining species, referred by Agassiz to Mactromya, are too imperfectly known to 

 justify us in pronouncing their true position with any confidence. 



Ceromya may now be considered as sufficiently established ; the hinge characters, which 

 were imperfectly known to Agassiz, have been fully described by M. Buvignier, ' Bull. Geol. 

 Soc. Fr.,' IS 50 ; for, although the shell upon which the latter author founded his description 

 is a Gresslya, we have ascertained that the hinges of the two genera are altogether alike. 

 M. d'Orbigny (' Prodrome de Paleontologie ') and M. Buvignier (' Paleont. Dep. de la 

 Meuse') have merged Gresslya in Ceromya, but we consider that Agassiz was justified in 

 regarding them as distinct, their figures are essentially different ; the Ceromyse are all 

 ventricose, with incurved equal subspiral umbones ; they are equivalve, for although there 

 is much irregularity in this respect, and occasional inequality of the valves, these variations 

 are altogether accidental, and resulted probably from the position which the Mollusk 

 occupied in the ground, or its proximity to other bodies ; their surface has regular ridges 

 which are not altogether smooth, they are concentric, or in other species they take an 

 oblique direction ; or, again, they suddenly change their direction and are reflected after 

 the manner of the Goniomyae. Gresslya, on the contrary, is never perfectly equivalve, the 

 right valve being always larger, and its umbo higher than the other ; the form is much 

 more compressed, the umbones more pointed, the surface is destitute of the peculiar ridges 

 of Ceromya, but possesses a different kind of ornamentation ; the outer layer consists of a 

 very delicate pellucid semicorneous test, with densely arranged radiating lines of granules, 

 the lines usually slightly undulate, and the granules, which are regular, are densely arranged, 

 and so minute as scarcely to be visible to the unassisted eye. M. Agassiz was not 

 acquainted with this fact, which we have ascertained by an examination of a large number 

 of examples in a good state of preservation. Cercoviya has been shown by M. d'Orbigny 

 to be identical with Anatina, of which it possessed the usual vertical fissure beneath the 

 umbones and the granulated surface ; but the aspect of the two forms differs in other 

 particulars, for Oolitic species are compressed, the posterior side is remarkably elongated, 

 and the anterior side has large longitudinal ridges. These features indicate a distinction 

 which we regard as of subgeneric value. We would, therefore, place Cercomya as a 

 sub-genus of Anatina. 



Goniomya is a form which we believe to be entitled to a separate generic rank, not- 

 withstanding M. d'Orbigny and M. Buvignier have reunited it to Pholadomya; the ridges 

 upon the sides are strongly impressed upon the internal casts, and are very different from 

 the costse of Pholadomya ; and it has, moreover, a granulated surface, the granules, as in 

 Gresslya. being radiating and linear. 



