GASTEROPODA OF 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 1s very closely related in general form to Lunatia Halli of 
the same author, but the flattening of the top of the yolution 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 conmon 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 Thill, 
New Jersey. Mr. Gabb’s types were from Crosswicks, New J ersey. 
GYRODES OBTUSIVOLVA. 
Plate xvi, Figs. 9-12. 
? Gyrodes obtusivolva Gabb: Proc. Acad. Nat. Sci. , Phila., 1861, p. 320, 
Gyrodes ? obtusivolva (Gabb) Meek: Check List Oret, and J ur. 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, Pl. r. Fig. 11. 
Shell, as known from internal casts, of. moderate size, somewhat crect, 
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 cle- 
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 XVILI——9 
