256 STEPHANOCERAS. 



of the saddle in ffoplites will almost always clear up all remain- 

 ing doubts. 



The general habit, whieh in most Haploceratites is easily 

 understood, it is scarcely possible to put into words, though I 

 will attempt in this relation to lay down some principles. A 

 large part of the forms is characterized by sickle-shaped fur- 

 rows, which, besides are found only in the genera Lytoceras and 

 Phylloceras, which are fundamentally different in their lobular 

 markings ; thin ribs, which are undivided and straight are also 

 confined to these genera. Regular and distinct division of the 

 ribs is never found in Haploceras. Slender, undivided, widely 

 separated ribs, hardly ever exceeding ten in number on one 

 whorl, are found only in Haploceras and in Lytoceras, which is 

 easily distinguished by its lobes. Also the alternation of strong 

 ribs with numerous finer ones, which are placed between the 

 coarser ; a prominent carina or a broad furrow are not present 

 on the external side. 



To define a genus in this way, may be considered very unpre- 

 cise and unscientific ; a proceeding hardly possible in any other 

 department of conchology, unless in the specimens under exam- 

 ination the most important parts are wanting ; in spite of this 

 meagre diagnosis, the species of Haploceras are however very 

 easily distinguished from their cotemporaries. 



The characters named up to this point are confined to the 

 geologically older forms ; a very peculiar development is assumed 

 by Haploceras in the upper strata of the cretaceous, in the 

 Turonian and Senonian, where they become developed into the 

 enormous giant forms of the group of the Hapl. peramplum ; 

 appearing at first as though not belonging here, but the agree- 

 ment in the lobes and inner whorls leaves little doubt that they 

 are to be here referred ; they are easily distinguished from all 

 other upper cretaceous forms by the lobes. 



Seventy-six species. Jura, Cretaceous. 

 HAPLOCERAS LIGATUM, d'Orb. T. 102, figs. 542, 543. 



Genus STEPHANOCERAS, Waagen. 



General form of the shell very variable, external side rounded 

 without keel, angle or furrow. Sculpture never sickle-shaped, 

 decorated with straight, bifurcating ribs, abundantly provided 



