PALAEONTOLOGY OF NEW-YOEK. 
Genus Amphigenia (nov. gen.). 
[ Gr. afx<pi, utrinque ; and yevea, generation 
Shells inequivalved, oval, ovoid or subtriangular, 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 cardinal process or hinge-plate, bor¬ 
dered by the teeth sockets, anchylosed 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 
prisnmtic 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 genus were referred by me to the Genus Rensselasria, for 
reasons already mentioned. They differ from that genus in the angular pit or 
trough made by the conjoined dental lamelke of the ventral valve, and (with our 
* The following remarks upon the dorsal valve of Rensseiaeria ( Pal. New-York, Vol. iii, pp. 454 
and 455) are precisely applicable to the dorsal valve of Amphigeria : “ In the dorsal valve, the dental 
“ sockets lie between the shell proper and a strong, often much, thickened process, from the anterior 
“ extension of which proceed the slender crural processes.” ***** 
“ The cardinal process at the base of the crura is often 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 distinctly marked by two grooves upon its summit. Behind this process, and between it and 
“• the beak, there is a distinct round foramen communicating beneath with the interior cavity of the 
“ valve.” 
