34 [Assembly 



the A. 'planoconvex a, both of the Clinton group*. The internal structure, 

 however, has been determined from the species of the Oriskany sandstone 

 and Lower Helderberg specimens, but mainly from the former. 



In its hinge-structure it approaches Tropidoleptus ; but the hinge-line 

 of the dorsal valve is never so far produced, while the beak is much more 

 extended. In Tropidoleptus there is a distinct linear area, and the teeth 

 and sockets are crenulated ; the form and character of the muscular im- 

 pressions are different, and the structure of the shell is punctate. 



Geological ra7ige. The genus begins its existence as low down in the 

 system as the Clinton group, and extends through all the members of the 

 series to the limestones of the Upper Helderberg, and perhaps above that 

 point. 



OBSERVATIONS ON THE GENUS EITONM. 



In the Annual Report on the Palaeontology of New- York for 1841, 

 Mr. Conrad described and figured a species of brachiopod from the 

 Oriskany sandstone under the name of Mrypa peculiarisf. A similar 

 species was described by Mr. Vanuxem, in his Geological Report in 

 1843, under the name of Atrypa singularisl, from the shaly limestone 

 of the Lower Helderberg group. 



These species are both remarkable in their form and exterior 

 surface markings; having one valve deeply sinuate on its anterior 

 margin, and the other with a long linguiform extension filling the 

 sinuosity, while the surfaces are finely marked by radiating strise. 

 A. third species, described by Mr. Vanuxem as the Atrypa medialis, 

 was placed in the same group with the two preceding, though the 

 external appearance would scarcely justify such an arrangement. 



In the course of continued collections in the Helderberg, nume- 

 rous casts of these species were obtained; and it became apparent 

 that they possessed a peculiar internal structure, leaving upon the 

 casts similar muscular and vascular impressions. For these forms I 

 proposed, in 1856, the name Eatonia||, describing several species. 

 The genus may be characterized as follows : 



* The last-named species presents some differences in the hinge-structure, which may prove 

 of sufficient importance to unite the lower group of species under another generic designation. 



•f Annual Report on the PalEBontology of New-York, 1841, pa. 56, pL f. 11. 



t Report on the Third Geological District of New-York, 1S43, p. 120, f. 3. 



II Report of the Regents of the University on the State Cabinet of Natural History for 1856 : 

 Also New Species of Paljsozoio Fossils, 1857. 



