INVERTEBRATE PALEONTOLOGY. 477 



TURRlLITID-ffi. 



Gcuii3 IIETEROCEItAS, d'Orbigny. 



Sifnon. — Tunititcs (sp.), of authors; not Lamarck. 



ffeteroceras, d'Orbigny (lti49), Conrs filem., I, 291; and (1850) Prodr., 102; also (1851) Jonr. 

 Conch. (Paris), 217.— Pictet.(1854), Traits do Paleont., II, 714.— Chenu (1859), Man. 



Couch., I, 96. — Meek (1SG4), Smithsonian (.'heck-List N. Am. Cret. Fossils, 25. 



Etym. — ercpoc, different ; kipag, a horn. 

 Type. — Heteroceras Emcricianum, d'Orbigny. 



Shell at first spiral, with volutions rounded, and contiguously coiled 

 around a rather large umbilical cavity; but at a later stage of growth with 

 the body-part tree, extended more or less nearly straight, or wilh a much 

 broader curve, away from the spire, and at last apparently with the free end 

 curving backward again, somewhat as in Ancyloceras ; surface ornamented 

 with annular costse, sometimes bearing nodes on the ventral or outer side of 

 the curve; siphon passing around the middle of the outer side; septa with 

 six branching, unequal lobes, and as many intervening sinuses, both of which 

 are unsym metrical in the spiral part, but nearly or quite symmetrical on 

 opposite sides of the siphuncle, as far as they extend along the deflected body- 

 part of the shell. 



Much uncertainty exists in regard to the limits of this genus, and its 

 relations to several allied type*. Formerly, the species were referred to 

 Turrilites, while even yet some include them in Helicoceras. They differ 

 from the former in having the volutions at last becoming free and deflected ; 

 while from the latter they differ in having them wholly or in part in contact 

 above the deflected part. They are also larger and more robust shells than 

 the types for which the genus Helicoceras was originally proposed, and have 

 the volutions not only in contact in the whole or a part of the spire, but 

 coiled so as to form a smaller, though large umbilical cavity. 



Again, this genus is allied to Helicaneyloceras, Gabb, from which it 

 appears to differ mainly in having a much more elevated spire, with volutions 

 more decidedly in contact. The great difficulty in distinguishing these groups 

 from each other is the fact that we nearly always have mere fragmentary 

 specimens 1o deal with. In fact, I am not sure that entire specimens show- 

 ing the whole shell, including the aperture with unbroken margins, of any of 

 these genera, have yet been found ; consequently, in most cases the charac- 

 ters of the entire shell have been partly inferred from detached portions, 

 supposed, from their ornamentation and association in the same beds, to belong 

 to different parts of the same types. 



