GENUS AMPHIGENIA. 383 



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 striata in precisely the same 

 manner. Renssel^eria belongs to the Family Terebratulidfe ; while Pentajierus 

 and Stricklaxdinia are of the Family Rhynchonellidae = Pentamerida, as usually 

 understood. The Genus Amphigenia, in its punctate shell and modification of the 

 hinge in the dorsal valve, offers absolute affinities with Rensseljeria, while in 

 other features it presents characters intermediate between the Pentamerida3 and 

 Tercl;ratulida3. 



Auipliigenia elongata. 



PLATE LIX. 



Pentamerut elongatut : Vanuxem, Report Third Geol. Dist. New-York, p 132. 1842. 



•< " Geol. Report Fourth Dist. New-York, p. 34. 1848. 



Mtganttrit elongatut : Hall, Tenth Report on the State Cabinet, p. 123. 1857. 

 Jlensselaria 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, 1851. 



" " 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 a]«ng 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 strias 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 



