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Beside the type form, Didymcoeras 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 Newtoni, Exploration of the Black Hills, PI. 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 Nostoceras helicinum, and these make 

 it necessary to study the young before it can be definitively referred 

 to the same genus with Nebrascense. 



Didymoceras nebrascense. 



Heteroceras nebrascense, Meek {Invert. Foss.,V\. xxii, Fig. 1). 



Heteroceras, Whitf. {Pal. Black Hills, PI. xv, Fig. 6). 



Loc, Near Buffalo Gap, S. Dakota. 



PI. xiv, Figs. 



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 costae increase in size and 

 prominence in the anageroritic substage, and also become tubercu- 

 lated and bifurcated. During the metagerontic substage these 

 characters are more developed, and the volution makes a retroversal 

 bend. All of the ornaments are lost, however, in the paragerontic 

 substage, the costae depressed and finally disappear except on the 

 venter, and the whorl becomes again bilaterally symmetrical. The 

 costae 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 tortum, and 



*Proc. Colorado Scien. Soc, ii, Pt._iii, 1S87. 



