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The species are as follows : Exiteloceras (Heteroceras) Cheyen- 

 nense and angulatum. Exiteloceras (Ancyloceras) uncum, Meek, 

 Invertebrate Paleontology, PL xxi, is probably a fragment of the 

 gerontic stage of one of these. 



Exiteloceras {Ancy loc eras) Jenny i, Whitfield, Paleontology of the 

 Black Hills, PL xvi, has also similar ornamentation, but the costse 

 differ somewhat. This form, if the drawing is correct, has a ten- 

 dency to asymmetry and when older was probably helicoidal. 



Ancyloceras lineatus, Gabb, Paleontology of California, has also 

 similar costse, form and tubercles, but this may be a fragment of 

 Ptychoceras. 



I have also before me two fragments, one 20 mm. in transverse 

 diameter by 19 mm. ventro-dorsally, the other 17 mm. transversely 

 by 20 mm. ventro-dorsally, which have precisely the costse and 

 tubercles of Exiteloceras angulatum , as figured by Meek, namely, 

 very prominent, subacute single costse reaching completely round 

 the whorl, each one having two tubercles on the venter with a slight 

 depression on the prominent costation between them. They are 

 fragments of helicoceran whorls and the aspect is altogether dis- 

 tinct from that of any form in other genera. Loc, Elm Fork, 

 Dallas county, Texas. Hamites Fremonti, Marcou, Geology of 

 North America, p. 36, PL i, Fig. 3, is probably a gerontic stage of 

 some species of this group. The anagerontic substage in his figure 

 has single costse without tubercles, but the metagerontic substage 

 has the retroversal bend and every third costation has two ventral 

 tubercles. All costse are single and prominent. 



Exiteloceras (Helicoceras) pariense, White, U. S. Geol. Survey 

 W. 100 Merid., Wheeler, Pt. i, Pal., PL xix, Fig. 2, is another 

 species of this series which shows by the twist in the costations that 

 it is probably in older stages helicoidal. 



Ptychoceratidoz. 



I use this family name here provisionally and only in order to 

 make clearer the essential distinctions that seem to exist between 

 the series represented by the genera, Sciponoceras, Ptychoceras and 

 Diptychoceras and other series of genera described in this paper. 



The young, so far as known, have slight, smooth shells in the 

 neanic stage, the ephebic stage has the lines of growth and cost?e 

 inclined forwards in passing over the sides and venter and probably 



