94 CARBONIFEROUS AMMONOIDS OF AMERICA. 



The whorl is low, broad, with trapezoidal cross section, very evolute, 

 broader than high ; indented about one-half the height by the preceding- 

 whorl. Greatest breadth at the umbilical margin, about three-fourths the 

 diameter; height of the whorl about two-sevenths the diameter and less 

 than half the breadth. Umbilicus broad and deep, width about one-half 

 the diameter. Strong umbilical tubercles, which are continued across the 

 abdomen by fine undulations. About three constrictions to a revolution. 

 Outer shell with fine cross strife, and obscure spiral stria? on the inner 

 whorls. Septa consisting of a pair of tongue-shaped narrow ventral lobes, 

 and a somewhat shorter and broader lateral lobe. On the umbilical 

 shoulders is a small shallow "suspensive" lobe. The saddles are all 

 broadly rounded. 



This species has been united by many writers with Gastrioceras car- 

 bonarkim, but is always broader and more depressed, has stronger umbilical 

 tubercles and broader lateral lobes than that species. Some have confused 

 it with G. marianum M. V. K., and the American representative has been 

 referred by the writer " to that species. But the figures and descriptions of 

 Foord and Crick enable these two species to be distinguished quite easily. 

 G. listeri is not quite so involute as G. marianum, is always broader and 

 coarser in sculpture. The breadth of the whorl in G. listeri is more than 

 three-fourths of the diameter, in G. marianum it is only two-thirds of the 

 diameter. Also in G. listeri the lobes are proportionally longer and 

 narrower. 



Occurrence. — In England, Belgium, and Germany Gastrioceras listeri 

 is characteristic of the middle division of the Coal Measures. In America 

 it has been found in the same horizon near Boles, Scott County, Ark., 

 accompanied by G. carbonarium. It may thus be taken as diagnostic for 

 this zone in these two regions. 



The figured specimens came from near Boles, Scott County, Ark., 

 and are deposited in the geologic collection of Leland Stanford Junior 

 University, California. 



"Proc. Am. Philos. Soc, Vol. XXXV, p. 260. 



