46 PALEONTOLOGY OF NEW JEKSEY. 



Genus LATIARCA Conrad. 

 LaUaren Conrad: Proc. Acad. Nat. Sci. Phil. 1862, p. 289. 



"Triangular, thick, capacious; hinge line narrow medially, broad and 

 thick on the sides; cardinal plates granular and laterally striated, towards 

 the ends in short oblique series; cardinal area wide with obliquely diverging 

 grooves. 



"X. (Cucullcea) gigantea Con., L. idonea Con., (1 ononrhehi Rogers, C. 

 transversa Rog'ers. (Eocene.)" 



I have serious doubts as to the identity, generically, of Area idonea 

 Conrad, with the Cneidkea gigantea — , and C. ouocliela of Kogei's (not 

 ononchela). The first one has a very strong internal muscular ridge 

 which always shows deeply on the internal casts, which none of the 

 specimens of A. idonea which I have seen possess; while (J. miochela of 

 Rogers possesses a hinge structure so entirely different from A. idonea that 

 it can not properly be considered as pertaining to the same group. Rogers's 

 species presents exactly the hinge features of Conrad's cretaceous genus 

 Idonearca, and the shell only lacks the strong muscular ridge on the interior 

 to make it exactly identical. In all other respects, both in the external 

 surface features, the ventricose form, wide area, thickened shell, and in the 

 general shape, they agree exactly; but A. idonea Conrad is entirely 

 different; it neither presents the hinged features, the finer striated exterior, 

 nor the dense and thickened shells of the Cretaceous forms ; and I think it 

 wrong to classify it with them. The Idonearcas are all Cretaceous; the C. 

 gigantea, C. onochela, and C. transversa (which presents the same features as 

 the C. onochela) are Eocene; while A. idonea has not been recognized out of 

 the Miocene deposits so far as I am aware. I think Mr. Conrad must have 

 been convinced of the error in this reference of A. idonea to Latiarca before 

 writing his list of Miocene fossils, in the later pages of the same volume in 

 which he describes Latiarca, as he there refers it to Scapharca, where it 

 more properlj^ belongs. The shells, however, show a very beautiful transi- 

 tion from those of the Cretaceous Idonearcas, tlu-ough the Eocene forms, 

 with and without the muscular ridge and finely striated surface, and strong 

 teeth, to those of the Miocene where the features are intermediate between 

 those and the present forms of our coast. 



