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 
gmm., 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 coste, 
each tuberculated on either side of the venter. “These become more 
or less irregularly bifurcated, and with intermediate entire coste 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- 
waras and outwards, as in the anagerontic substage of Vostoceras 
Stantont, var. aberrans. 
The single tuberculated coste of the young are similar to those 
of the later stages of Avzcyloceras Jennyt, 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,} Nn. g. 
There are aseries of forms having loose helicoid spirals, two 
rows of more or less irregular ventral tubercles and irregularly 
bifurcated coste, 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. 
t+4dvpog, double. 
