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rows of ventral tubercles, which become more or less deflected 
during development towards the lower side (whether this be the 
left or right side) of the whorls. There is a contact furrow which 
is maintained as long as the whorls are sufficiently close coiled and 
then disappears on the free gerontic volution. This volution is 
excentric and then recurved as in all retroversal living chambers. 
The nepionic stage is not yet known, but it is obviously quite 
different from that of the more loosely coiled helicoceran spirals of 
the genus Emperoceras. 
This genus has the two rows of ventral tubercles and general 
aspect of the shells of Ptychoceras and Emperoceras, described in 
this paper and these characters contrast decidedly with those of the 
Helicancylus phylum. 
The nepionic and perhaps earlier neanic substages are not known, 
but there are indications in some specimens of JVostoceras helicinum 
that in the early neanic substages the whorl is not a normal ammo- 
noidal spiral, but an open, whorled, irregular shell of some kind. 
The specimens I have in hand also show that in both species, 
Nostoceras Stantont and Nostoceras helicinum, the last neanic or 
earliest ephebic substage has no contact furrow and has single costz 
without tubercles. The ephebic stage has tubercles as a rule, and 
more or less bifurcated costz, but both may be absent in some shells. 
The gerontic volution is apt to have tubercles even when they are 
absent in the ephebic stage. 
This genus is of interest in connection with the history of the 
impressed zone because here, as in other allied forms, this charac- 
teristic enters upon a new phase of its history. The nepionic stage 
being unknown, one cannot state positively that it has a close-coiled 
shell and a contact furrow, but since this has now been found in so 
many uncoiled forms, it is legitimate to infer that it was present. 
In such shells the contact furrow which arises after the degeneration 
and loss of the nepionic contact furrow is obviously distinct, 
occupying the side and not the dorsum of the whorl. 
The type is WVostoceras Stanton’, U.S. National Museum; Loc., 
Chatfield, Novarro county, Texas. 
NOSTOCERAS STANTONI. 
Loc., Chatfield, Novarro county, Texas. 
This species has several varieties. 
Var. retrorsus. 
This variety has from five to six complete turrilites-like whorls 
