Maryi.axd Geological Survey 443 



extend far upward into the dorsal fold. The median portion of the dorsal 

 valve is elevated or ridged and toward the anterior margin may or may not 

 bend ventrally, but usually remains nearly plane when the ventral shell 

 extension rises to meet this ridge. This is the M. arcuata group. Besides 

 this species there are in the Helderberg and Oriskany M. arcuuta aioha, 

 M. hlancha, M. lata, M. lenta, M. lentiformis, M. prcenuntia (Keyser), 

 M. piinceps, M. rostellata, M. subquadrata, and M. ( ?) vascularia. 



The second section is devoid of the dorsal arcuation and the ventral 

 sinus is rarely well developed. This is the M. la:vis group and embraces 

 M. hella, M. Icevis, M. symmetrica, M. walcotii, M. gigantea, and M. 

 ivhitfieldi. 



Mehistella prjenuntia n. sp. 

 Plate LXXIII, Figs. 43-46 ; Plate LXXIV, Figs. 1-4 



Description. — This species is prenuntial of M. arcuata Hall and J/. 

 arcuata atoka Girty ' of the New Scotland member. Its variations are 

 many — even forms with rudimentaiy plications occur — but none appear 

 from which M. Icevis could have developed. All are somewhat arcuated 

 and an occasional specimen is present that could be readily regarded as a 

 small M. arcuata. However, the many variations of M. prcenuntia, all of 

 which are united by intermediate forms, pre\ent the reference of this 

 Keyser material to M. arcuata. 



The characters of M. prcenuntia, when contrasted with M. arcuata, are 

 uniformly smaller growth, less dorsal arcuation almost always greater 

 elongation (often there is a linear dorsal sinus extending almost to the 

 beak), and the presence in many specimens of rudimentary plications. 

 The ventral sinus is also apt to be angulated or sharply bounded laterally, 

 sometimes quite so, and in such specimens the dorsal linear sinus is also 

 more prominent, causing the anterior margin to be slightly notched. 

 The angulation of the ventral sinus is carried to an extreme in the Indian 

 Territory M. arcuata atolca, which has a decidedly notched anterior 

 margin. 



' Nineteenth Ann. Rept. U. S. GeoL Surv., 1899, p. 567, pi. Ixxi, figs. la-1/. 



