INVERTEBRATE PALAEONTOLOGY. 485 



they l>e found attaining as large a size, and in such short fragments as not to 

 show the exact nature of the curve. Like the last, it is only referred provis- 

 ionally to this genus upon the supposition that it belongs to the deflected part 

 of the body of the shell. 



Locality and position. — Head of South Branch of Cheyenne River, 

 Dakota; from the upper part of the Fort Pierre group of the Upper Mis- 

 souri Cretaceous series. 



Genus HELICOCERAS, d'Orbigny. 



Synon. — Turrilites, Hamites, &c. (sp.), of authors. 



Helicoceras, d'Orbigny ( 1840), Pal<5ont. Fr. Terr. Cre~t., 1, 611 ; and (1850) <&., Terr. Jurass., I, 598 ; 

 also (1850) Prodr., I, 263 ; and ib., II, 103 and 127.— Morris (1843), Cat. Brit. Fossils, 

 181.— Baugier and Sauz<5 (1843), Notice sur Quelques Coii., 15.— Deshayes (1845) iu 

 Lamarck (2d ed.), XII, 266.— Quenstedt (1852), HaDdb. der Petref. (index), 772.— 

 Pictet (1854), Traite Paleont. II, 713.— Chenn (1859), Man. Conch., 1, 96.— Meek and 

 Ilayden (1860), Proceed. Acad. Nat. Sci., XII, 21.— Meek (1864), Smithsonian Check- 

 List N. Am. Cret. Fossils, 25.— Gabb (1864), Palajont. California, I, 71. 

 Belicocerus, King (1844), Ann. Mag. Nat. Hist., XIV, 278. 



Etijm. — eAif, twisted ; a'cpac , a horn. 

 Kxamp. — Helicoceras annulatitm, d'Orbigny. 



Shell slender, terete, spiral ; volutions few, widely disconnected, and 

 coiled so as to leave a very broad umbilical space within, and form a much 

 depressed spire; surface ornamented with simple, or bifurcating, annular 

 costre, which are either continuous, or interrupted on the siphonal side ; 

 septa each provided with six unequal, branching, and digitate lobes, the 

 second lateral and antisiphonal ones of which are often divided into impaired 

 branches, while the six intervening sinuses have their branches in pairs ; 

 siphuncle on the outer or ventral side. 



As here defined, the genus Helicoceras includes two sections that may 

 be distinguished as stated below : 



1. helicoceras, d'Orbigny (typical). 



Shell with the annular costse passing uninterruptedly over the 

 rounded siphonal side. — (H. annulatum, d'Orbigny.) 



2. patoceras," Meek. 



Shell differing from the typical form in having the costaj sud- 

 denly and completely interrupted on the siphonal side, so as to leave 

 a narrow, smooth, path-like space along the whole length of the 

 same. — (H. Teilleu.ci, Baugier.) 

 Pictet and some others also refer to this genus Turrilites Robertiauus, 



* jrQTrf. a path (iu allusion to the suiuo'h path-like space along its siphonal side) ; aipac, a horn. 



