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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 Nostoceras 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 Stantoni and Nostoceras helicinum, the last neanic or 

 earliest ephebic substage has no contact furrow and has single costse 

 without tubercles. The ephebic stage has tubercles as a rule, and 

 more or less bifurcated costae, 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 Nostoceras Stantoni, 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 



