54 



PROCEEDINGS OF THE ACADEMY OF 



IDONEARCA. 



Shell triangular, thick, ventricose, with radiating lines, hinge 

 broad, medial cardinal plates prominent, linear, transverse, or di- 

 rect and transversely striated ; anterior and posterior plates elon- 

 gated, oblique or descending, angular at the inner ends and 

 strongly striated transversely, inner plate prominent, curved, car- 

 dinal area subequal, grooved. 



I. capax. PI. II., fig. 2. Journ. Acad. Nat. Sci., 2d series, vol. III., p. 328, pi. 

 XXXV., fig. 2. 



This genus is one of the most characteristic of the Cretaceous 

 forms, and has the earliest and most developed form of hinge, now 

 represented by the recent Cuculleea. Stoliczka 'confounds with 

 Macrodon, the two very distinct genera of Idonearca and Gram- 

 matodon. The latter is a Jurassic genus unknown in Cretaceous 

 strata. 



Having obtained the hinge of a New Jersey bivalve belonging 

 to a group of which Cuculleea vulgaris, Morton, is the typical form, 

 I am enabled to subjoin a list of all the species of Idonearca 

 known to me. 



American. 



antrosa, Morton. 

 capax, Conrad. 

 Mattliewsoni, Gabb. 

 Tippana, Conrad. 

 truucata, Gabb. 



Syrian. 

 brevifrons, Conrad, 

 induratus, Conrad, 

 opiforrnis, Conrad, 

 orientalis, Conrad, 

 sabrotunda, Conrad. 

 Syriaca, Conrad. 



European. 



glabra, Sowerby. 

 disparilis, 2)' Orbig. 

 Gabrielis, D : Orbig. 

 fibrosa, D 1 Orbig. 

 Marciana, D, Orbig. 

 sagittata, D' Orbig. 

 tumida, B'' Orbig. 



Indian. 

 disparilis, W Obigny. 



TRIGONOARCA. 



This genus is eminently characteristic of the Newer Cretaceous 

 series. It has very marked and distinct generic characters, par- 

 ticularly in having teeth like Axincea and an internal elevated 

 plate like Idonearca and Cucull^a. 



Stoliczka doubts whether this genus should not rather be a sub- 

 genus of Noetia, a very strange reference considering that Noetia 

 is strongly ribbed, has comparatively fine hinge teeth, no internal 

 plate and reversed beaks, and above all is no older than the Mio- 



[June 25, 



