579 
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. Iam 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 Limperoce- 
vas Beecheri in the singleness and tuberculation of each one of the 
cost. 
PTYCHOCERAS CRASSUM, Whitfield. 
Loc., Near Boulder, Colo., Cretaceous. 
Pl. xiv, Figs. 18-21. 
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 costz 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 costz after this become more 
