﻿573 



NOSTOCERAS HELICINUM. 



Heteroceras helicinum, Shumard.* 

 Loc, Chatfield, Novarro county, Texas. 



At the diameter of 8 mm. in one of the two specimens before me, 

 there are indications that the young was more loosely coiled, and 

 perhaps more or less excentric in comparison with the later closer- 

 coiled stages. The contact furrow was also obviously absent in 

 these earlier substages. In the other specimen, at diameter of about 

 9mm., there are similar indications. Nevertheless, I was by no 

 means sure of what these changes indicated, whether a helicoceran, 

 scaphetoid or hamites-like shell. All that can be said is that they 

 show irregularities in the growth of the young not present in the 

 turrillitean volutions of the ephebic stage. 



The young, probably in the anephebic substage, has single costse, 

 each tuberculated on either side of the venter. These become more 

 or less irregularly bifurcated, and with intermediate entire costse with- 

 out tubercles, usually one, sometimes two, in each interspace in the 

 metephebic substage. The whole is a flat turbinated coil of not 

 more than four or five whorls with prominent tubercles and costa- 

 tions. 



In the anagerontic substage the volution abandons the spiral, the 

 contact furrow disappearing immediately, and the shell grows down- 

 wards and outwards, as in the anagerontic substage of Nostoceras 

 Stantoni, var. aberrans. 



The single tuberculated costae of the young are similar to those 

 of the later stages of Ancyloceras Jennyi, Whitf., Pal. Black Hills, 

 and some of the helicoceran forms found elsewhere ; but the young 

 shells were obviously quite different, being more closely coiled and 

 stouter shells. Specimen in Coll. U. S. Nat. Mus., No. 21103. 



Didymoceras,\ n. g. 



There are a series of forms having loose helicoid spirals, two 

 rows of more or less irregular ventral tubercles and irregularly 

 bifurcated costae, which also have, or appear to have, a gerontic 

 stage with a retroversal volution, as in Nostoceras. These are all 

 larger shells and are separable by the helicoceran mode of growth 

 in the ephebic stage. 



♦Received through the kindness of Mr. T. W. Stanton, who identified the species. 



■\A{dup.o$, double. 



