486 UNITED STATES GEOLOGICAL SURVEY OP THE TERRITORIES. 



d'Orbigny, which was only known to the latter author in the condition of 

 fragments, from which he was led to regard it as having its volutions in con- 

 tact, as in Turrilites. Pictet, however, has figured a specimen, referred by 

 him to d'Orbigny's species, that shows the volutions to be clearly discon- 

 nected, more nearly as in Helicoceras. If it really belongs to this genus, 

 however, it would represent a third, very distinct section, on account of its 

 more closely-coiled volutions, more produced spire, and particularly in conse- 

 quence of having the costse that cross its siphonal side, with nodes placed 

 between them, so as to form three longitudinal rows along this outer surface. 

 It is also a much larger and more robust shell than d'Orbigny's types of 

 Helicoceras. 



Owing to these differences, I do not feel warranted in including this 

 shell in the same genus with d'Orbigny's original typical forms of Helicoceras. 

 It seems to me rather to form the type of a distinct genus; but whether this 

 view is correct, or it ought to be included as a section of that genus, it should 

 most probably receive a distinct name. Hence, I would propose to call the 

 group into which it falls Spiroceras. 



From the foregoing, it will be seen that I adopt this genus in its most 

 restricted sense, as represented by d'Orbigny's original typical species H. 

 annulatum and II gracile. Mr. Daniel Sharpe proposed, in his valuable 

 memoir on the English Chalk Cephalopoda, to extend its characters so as to 

 make it include a part of the species usually referred to the genus Turrilites, 

 notwithstanding the fact that these shells (d'Orbigny's section Rotundati of 

 the genus Turrilites) have their volutions in contact. They differ, however, 

 from the other section of that genus (the Angulati), not only in their shorter 

 and broader form, wider umbilicus, and rounded volutions, but in having their 

 siphon on the outer side of the curve, instead of near the suture above ; 

 while none of them are known to have their aperture hooded, as in the other 

 section. Still, they differ widely from Helicoceras as here understood, and 

 as originally founded by d'Orbigny. 



Although I at one time fully adopted Mr. Sharpe's views in regard to 

 these shells, and still concur with him in the opinion that they should most 

 probably be separated entirely from the genus Turrilites, as represented by 

 d'Orbigny's group Angulati, a further consideration of the subject has led me 

 to doubt the propriety of including them in the same genus with those little 

 shells such as d'Orbigny originally founded the genus Helicoceras upon ; char- 



