46 PALEONTOLOGY OF NEW JERSEY. 
Genus LATIARCA Conrad. 
Latiarca 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. 
“LT. (Cucullea) gigantea Con., L. idonea Con., C. ononchela Rogers, C. 
transversa Rogers. (Eocene.)” 
I have serious doubts as to the identity, generically, of Arca adonea 
Conrad, with the Cucullea gigantea —, and C. onochela of Rogers (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 C. onochela 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 
Idonearea, 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. zdonea 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 properly belongs. The shells, however, show a very beautiful transi- 
tion from those of the Cretaceous Idonearcas, through 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. 
