574 
Beside the type form, Dédymcoeras nebrascense, in the Yale Uni- 
versity Museum, there are several closely allied species, as follows: 
Didymoceras (Het.) cochleatum and tortum, Meek, Invertebrate 
Paleontology, Pl. xxi. 
Some of the species described by Whitfield and others from the 
Black Hills have similar ornamentation and helicoceran whorls, and 
probably belong to the same series, if not this genus. I refer to 
Heteroceras Newtont, Exploration of the Black Hills, P\. xv, and 
its possible gerontic stage, Ancyloceras tricostatus, Fig. 7 of same 
plate. With regard to this form I have, however, doubts arising 
from its close resemblance to /Vostoceras helicinum, and these make 
it necessary to study the young before it can be definitively referred 
to the same genus with WVedbrascense. 
DIDYMOCERAS NEBRASCENSE. 
HETEROCERAS NEBRASCENSE, Meek (/aver¢. Foss., Pl. xxii, Fig. 1). 
HETEROCERAS, Whitf. (Pa?. Black Hills, Pl. xv, Fig. 6). 
Loc., Near Buffalo Gap, S. Dakota. 
IP) xa er igs. 
The young of this species is unknown, but the younger stages of 
the closely allied cochleatum show that it did not have a contact 
furrow—at least in the early ephebic substage. The metephebic 
substage has more or less irregular, obscure tubercles and rather 
fine, closely set costae, occasionally bifurcated at the tuberculations. 
These disappear in the parephebic substage. The geronic volution 
is retroversal as in Nostoceras. The costz increase in size and 
prominence in the anagerontic substage, and also become tubercu- 
lated and bifurcated. During the metagerontic snbstage these 
characters are more developed, and the volution makes a retroversal 
bend. All of the ornaments are lost, however, in the paragerontic 
substage, the costa depressed and finally disappear except on the 
venter, and the whorl becomes again bilaterally symmetrical. The 
coste and lines of growth bend slightly forwards across the venter, 
then backwards into sinuses on the inner parts of either side and 
form symmetrical crests across the dorsum. The aperture is pre- 
served in this specimen and shows the same outline. 
Mr. T. W. Stanton* in discussing a collection of fossils from Fort 
Pierre shales, near Boulder, Colo., described substantially the 
.same remarkable characteristics in this species and in fortum, and 
*Proc. Colorado Scien. Soc., ii, Pt. iii, 1887. 
