GASTEROPODA OP THE LOWER GREEN MARLS. 129 



volution bordering the umbilical opening obsoletely angular or very sharply 

 rounded; aperture elongate-ovate, widest below the middle; surface un- 

 known. 



This species is very closely related in general form to Lunatia Halli of 

 the same author, but the flattening of the top of the volution separates it 

 from that genus, and places it in Gyrodes, as intimated by the author in the 

 original description of the species. It is much less common than L. Halli 

 and smaller in size, besides having a lower spire, and is perhaps a little less 

 erect, although in this latter feature they are so nearly alike that it is diffi- 

 cult to pronounce between them. 



Formation and locality: In the Lower Green Marls at Mullica Hill, 

 New Jersey. Mr. Gabb's types were from Crosswicks, New Jersey. 



Gyrodes obtusivolva. 



Plate XVI, Figs. 9-12. 



? Gyrodes obtusivolva Gabb: Proc. Acad. Nat. Sci., Phila., 1861, p. 320. 

 Gyrodes ? obtusivolva (Gabb) Meek: Check List Cret. and Jur. Foss., p. 1. 

 Gyrodes obtusivolva (Gabb) Meek: Geol. N. J., Newark, 1868, p. 729. 

 Lunatia obtusivolva (Gabb) Conrad: Am. Jour. Conch., vol. 5, p.45, PI. i, Fig, 11. 



Shell, as known from internal casts, of moderate size, somewhat erect, 

 obliquely subglobose with a moderately elevated spire, whorls three or 

 three and a half, the outer ones flatly truncate on the top adjacent to the 

 suture line, the truncation being strongly marked and angular at the mar- 

 gin. On fully grown specimens it is nearly an eighth of an inch in width 

 on the outer half of the last volution; aperture oblique, ovate, widest below 

 and truncated above by the flattening of the upper surface of the volution; 

 umbilicus, as seen in the casts, small, indicating a slender, almost if not 

 entirely solid columella; margin of the umbilical depression not angular; 

 surface of the shell, as seen on fragments remaining attached to the casts, 

 marked by fine transverse lines of growth. 



This species is closely allied to G. petrosa Morton's sp., but is more 

 erect, less spreading or patulose on the last volution, and has a more ele- 

 vated spire and smaller umbilical cavity in the cast. As seen in its usual 

 condition it is intermediate in form between G. petrosa and Lunatia Halli 



MON XVIU 9 



