INVERTEBRATE PALAEONTOLOGY. 487 



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acterized by very slender, widely-disconnected volutions, forming so broad a 

 curve as to give origin to a very wide umbilical cavity. As all of these 

 generally much larger shells, with rounded, connected volutions, forming 

 d'Orbigny's section Rotmidati of the genus Turrilites, arc, I believe, yet 

 only known in the condition of specimens with the terminal portion broken 

 away, it seems quite as probable that they may, when entire, have the body- 

 part deflected so as to conform to the characters of the genus Heteroccras. 

 Consequently, I think we are not justifiable yet in making Helicoceras so 

 comprehensive as proposed by Mr. Sharpe. 



Dr. Stoliczka gives this genus the same limits that Mi - . Sharpe did, and 

 yet makes the singularly inconsistent remark that, " when d'Orbigny first 

 established the genus Helicoceras (Pale"ont. Fr., I, 611), he described under 

 it two species, which very probably do not belong to this genus, and which 

 may prove to be coiled fragments of Anisoceras.''* He therefore seems to 

 ignore the fact that these two species which he thinks may " not belong to 

 this genus " are the typical forms on which it was alone founded originally ; 

 and thai the name must always adhere to the group to which they belong, 

 and cannot be transferred to a different group, even if d'Orbigny and others 

 did, at later dates, include in it species of this extraneous type. If d'Orbigny's 

 original types are congeneric with the group for which Pictet subsequently 

 proposed the name Ariisoceras (which may or may not be the case), then the 

 latter name would be only a synonym of Helicoceras: 



As here understood, the genus Helicoceras seems to have been intro- 

 duced during the Oolite period, ranges into the Cretaceous, and, if the fol- 

 lowing-described species really belongs to this genus, it must have continued 

 on until the deposition of the upper part of the latter series. 



Helicoceras Mortoni, var. tciiiiicostatiiin. 



Plato 22, figs. 3, a, b, c. 



Hamites Mortoni, Hall ami Meek (1854), Meru. Am. Acad. Arts and Sci. Boston, V, 396, pi. iv, fig. 3. 

 Ancyloceras, Hall arid Meek, ib., 411. 



Helicoceras temticostatum, Meek and Hayden (1858), Proceed. Acad. Nat. Sci. Pbilad., X,56. 

 Helicoceras Mortoni, Meek (1864), Smithsonian Check-List N. Am. Cret. Fossils, 25. 



The only specimens of this species that have yet been seen are frag- 

 ments, the largest of which is 1.39 inches in length, with a diameter of 0.49 

 inch at one end, and 0.44 inch at the other. It makes a broad, sinistral, 



* Palajont. Indica, I, 182. 



