568 
which may be new. The gerontic stage is the same as that of 
fT. equicostatum, and is closely bent and like Ptychoceras, although 
there is no gerontic contact furrow. The coste are large, single 
and tuberculated, not alternately entire and tuberculated as in 
Gabb’s species. 
LNVostoceratide. 
This is probably a more or less artificial group, but it serves the 
present purpose of showing the common characteristics of several 
groups of phylogerontic species. I have united under this name all 
such distorted forms of the Cretaceous in this country with unsym- 
metrical spirals in the ephebic stages, more or less prominent cost 
and two rows of tubercles on the abdomen. The earliest stages are 
too little known for any general description to be given, the 
-gerontic stages often have a retroversal living chamber and are 
tuberculated. ’ 
The genera are Nostoceras, Didymoceras, Emperoceras, Exitelo- 
ceras. 
Quenstedt was the first to call attention to the persistency of 
styles of ornamentation in series of degenerative shells and to point 
out that these were indications of affinity that could not be lightly 
laid aside. A considerable proportion of the phylogerontic species 
of the Cretaceous in this country have only two rows of tubercles 
with costations bifurcated at the bases of these tubercles, but I have 
not been able to find any corresponding ornamented normal form 
which might be considered their phylogerontic radical. 
flelicoceras Stevensoni (Whitfield) is represented among the 
specimens sent me from Yale Museum by a fine specimen and the 
youngest part of this specimen indicates a change in the spiral, 
but the young was not sufficiently defined to enable me to place the 
species in its proper genus. 
I have before mea fragment of a whorl very similar to Stevensoni 
in coste and tubercles, but of larger size than is usual in that 
species, and yet this has an irregular contact furrow on the upper 
side. The irregularity of this furrow may be due to age and the 
species may have been a true turrilites-like form when younger, or 
it may indicate that the separation of shells with the helicoceran 
mode of growth into different genera from true turrilites-like shells 
with a contact furrow is artificialand not advisable. This fragment 
