GENUS AMPHIGENIA. 
083 
present knowledge) in the free crura of the dorsal valve without apparent appen¬ 
dage. The muscular impression in the dorsal valve is precisely alike in the two 
genera, and the shell is punctate and externally striate in precisely the same 
manner. Rensseljeria belongs to the Family Terebratulidee ; while Pentamerus 
and Stricklandinia are of the Family Rhynchonellidse = Pentameridae, as usually 
understood. The Genus Amphigenia, in its punctate shell and modification of the 
hinge in the dorsal valve, offers absolute affinities with 'Rensselairia, while in 
other features it presents characters intermediate between the Pentameridse and 
Terebratulidae. 
Ampfliigenia elongata. 
PLATE LIX. 
Pentamerus elongatus : Yanuxem, Report Third Geol. Dist. New-York, p. 132. 1842. 
“ “ Geol. Report Fourth Dist. New-York, p. 34. 1848. 
Meganteris elongatus : Hall, Tenth Report on the State Cabinet, p. 123. 1857. 
Rensselceria elongata : Id. Pal. New-York, Vol. iii, p. 453. 
“ « “ Twelfth Report on the State Cabinet, p. 37. 1859. 
Stricklandia elongata : Billings, Canadian Journal, No. xxxiii, p. 268. Mav, 1861. 
“ “ Id. Geology of Canada, p. 371. 
Shell elongate-oval or ovate, subcylindrical, more or less convex or 
gibbous, variable in form, sometimes nearly as wide as long; the 
sides curving or nearly straight; front rounded or subtruncate. 
Ventral valve usually the more convex, often abruptly elevated or 
obtusely subangular along the middle ; umbo prominent : beak 
abruptly attenuate and closely incurved over the umbo of the oppo¬ 
site valve. 
Dorsal valve more or less convex or sometimes gibbous in the upper 
part, often more prominent or subangularly elevated along the middle 
of the upper part, and depressed-convex towards the sides and on the 
lower part, without evidence of mesial fold or sinus. 
Surface covered by regular radiating flattened striae, with fine concen¬ 
tric lines which are often crowded into squamose imbricating ridges 
of growth. When partially exfoliated, the radiating striae are only 
obscurely or not at all visible, and the concentric striae appear to be 
the external markings of the surface. Entire shell-structure punctate. 
This fossil is extremely variable in form; in the young state it is 
often as wide as long or wider, the hinge-line extended, and the greatest 
