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The extremely attenuated and much elongated cone of this spe- 

 cies is also altogether different from that of Ptychoceras. 



Another very interesting line of investigation is suggested also 

 by these studies. The resemblances of this shell and the full-grown 

 and senile stages of Baculites are indications either of affinity or 

 very close morphic parallelism. I am personally inclined towards 

 the latter opinion since Baculites itself seems to me to be a com- 

 posite of the extreme phylogerontic forms of several different 

 genetic series. 



Ptychoceras. 



This genus is interesting here on account of its relations to the 

 gerontic stage of Helicancylus and also because of the presence of 

 a secondary development of the impressed zone, which appears 

 during the gerontic stage of the ontogeny of most of the species 

 now referred to this genus. This last character and the close 

 angular bending of the straight limbs of the whorl, separate the 

 species wherever they occur. It is very likely that eventually this 

 and Baculites will be split into distinct series and shown to belong 

 to a number of different genera, but just now, with the exception 

 of Sciponoceras, this is not desirable. 



The American fossils I have seen all have the two rows of ventral 

 tubercles and are not similar to the Helicancylus phylum in their 

 ornamentation. They are much more like the young of Emperoce- 

 ras Beecheri in the singleness and tuberculation of each one of the 

 costse. 



Ptychoceras crassum, Whitfield. 



Loc, Near Boulder, Colo., Cretaceous. 

 PI. xiv, Figs. 1 8-2 1. 



Fragments Of this species kindly loaned me by Mr. T. W. Stan- 

 ton and identified by him show the following significant facts in 

 the history of the impressed zone. The ephebic stage, which is I 

 think the latter part of the straight arm with closely set tubercu- 

 lated costse inclined orally, is of the usual rounded form in section. 

 The venter between the two rows of ventral tubercles is narrower 

 than the dorsum, which is somewhat flattened but still entirely gib- 

 bous, the ventro-dorsal and transverse diameters are nearly equal 

 near the bend. As the bend is made a decided enlargement of the 

 ventro-dorsal diameters occurs and the costae after this become more 



