Fauna of the Morrow Group 
181 
the valve to the anterior margin slightly flattened transversely, 
umbonal region broadly convex, the posterior lateral edges of 
the valve deflected to form a sort of false cardinal area; beak 
broad, projected conspicuously beyond that of brachial valve, 
truncated by a large circular foramen ; anterior margin truncated 
by sinus of opposite valve. Internally, the dental lamellae are 
well developed. 
Brachial valve much shallower than pedicle, umbonal region 
low and broad, surface of valve in front of umbo distinctly flat- 
tened transversely except near the margins ; beak low and incon- 
spicuous, incurved beneath that of opposite valve ; a mesial sinus 
originating at about the mid-length of the valve and becoming 
broader and deeper anteriorly with broad rounded transverse 
outline and with its floor regularly increasing in convexity toward 
the front. Internally, the median septum is very short and the 
dental lamellae are well-developed, diverging rapidly from the 
hinge sockets. 
Surface of internal cast smooth, except for one or two growth 
lines near the margin. Shell structure finely punctate. 
Remarks. This species is represented by three internal casts, 
two of which are nearly complete. It is probably not a Girty^ 
ella as it lacks the strongly developed median septum of that 
genus but it cannot be referred to any other of the loop-bearing 
genera. The form seems to represent a new genus but that point 
could not be determined without more material than is now at 
hand. 
Horizon and locality. Kessler limestone: East Mountain, 
Fayetteville, Arkansas (Station 209). 
SPIRIFERID^ 
Genus SPIRIFER Sowerby 
Spirifer rockymontanus Marcou 
■ Plate XII, figures 1-6. 
1858. Spirifer rocky-montana. Marcou, Geol. N. A., p. 50, pi. 7, figs. 4-4e. 
Mountain limestone: Tigeras, New Mexico. 
1860. Spirifer subventricosa. McChesney, Desc. New Pal. Foss., p. 44. 
Coal Measures: Big Creek, near New Harmony, Indiana. 
1861. Spirifer Rocky montani. Newberry, Iveses Colo. Expl. Exped., p. 127. 
Upper Carboniferous limestone: New Mexico. 
