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which may be new. The gerontic stage is the same as that of 

 H. cequicostatum, and is closely bent and like Ptychoceras, although 

 there is no gerontic contact furrow. The costas are large, single 

 and tuberculated, not alternately entire and tuberculated as in 

 Gabb's species. 



Nostoceratidce. 



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 costse 

 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. 



Helicoceras 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 me a fragment of a whorl very similar to Stevensoni 

 in costse 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 artificial and not advisable. This fragment 



