3S« PALEONTOLOGY OP NEW-YORK. 



GeXVS AmPIIIGEXIA (nov. gen.). 



[ Gr. Ofi^i, utrinque ; aud yevca, feneratio.} 



Shells inequivalved, oval, ovoid or feubtriangular, more or less convex 

 or gibbous, ■without mesial fold or sinus. Valves articulating by teeth 

 and sockets, without area. The dental lamellae in the ventral valve 

 conjoined on their dorsal sides, forming an angular trough or pit, 

 which opens exteriorly by a triangular fissure beneath the beak, and 

 in its anterior extension is supported on a central septum. Dorsal 

 valve with a strong thickened cardyial process or hinge-plate, bor- 

 dered by the teeth sockets, anchjdosed to the bottom of the valve and 

 supporting the crura, which extend into the cavity of the shell.* 

 In all the specimens examined, there is a foramen extending beneath 

 the hinge-plate, and ending in a perforation at the beak. The shell sub- 

 stance is distinctly punctate, and, in exfoliated specimens, presents a 

 prismatic structure. So far as known, the crura terminate in slender 

 extensions, without appendage of any kind ; but we have had no speci- 

 mens in a condition to enable us to ascertain their entire character. 



The type of this genus, T. elongata, has been placed by Mr. Billings 

 under ihe Genus Stricklandia = Stricklandinia, from which I regard it 

 as differing in having no area on the ventral valve, a more extended 

 angular pit, a very different hinge-plate, with a foramen in the dorsal 

 valve, and a punctate shell-structure. 



The shells of this geuus were referred by mc to the Gcuus Kexssel-^kia, for 

 reasons already nieiitioned. They differ from that gonus in the angular pit or 

 Iroisgh made by the conjoined dental lamellae of the ventrsiV valve, and (with our 



• The following remarks upon the dorsal valve of Kbnssel:f.ria ( Pal. New- York, Vol. iii, pp. 454 

 and 455) are precisely applicable to the dorsal valve of Amphiobnia : " In the dors.il valve, the dental 

 " sockets lie between the shell proper and a strong, often mnch thickened process, from the anterior 

 " e.Ytansion of which proceed the slender crural processes." * • • * • 



" The cardinal process at the base of the crura is mClcn much thickened, and sometimes extends for- 

 " ward into the shell much more than in others ; and, when it becomes thickened in old shells, is 

 " often di-stinctly marked by two grooves upon it.^ summit. Behind this jirDcess, and between it and 

 " the beak, there is a distinct round foramen communicating beneath with the interior cavity of the 

 " Talve." 



