140 PALAEONTOLOGY OF CALIFORNIA. 



Length of the largest fragment, 2.7 inch; greater diameter, .85 inch; lesser, .7 

 inch ; distance between the ribs, .1 inch. 



One specimen from the Shasta Group (?) Cottonwood Creek, another from near 

 Folsom, from " Fiock Corral," found by Mr. Gorham Blake. 



HELICANCYLUS, N. Gen. 



General form resembling Ancyloceras; shell commencing with. 

 a dextral, open, descending spiral, as in Helicoceras, the spiral 

 afterwards opening, the shell becoming straight for a definite 

 distance, and then recurving as in Ancyloceras. 



The relations of this genus are obvious; it is an Ancyloceras in which the spiral 

 portion, instead of forming in the same plane, descends as in Helicoceras. 



It may be questioned, how far these aberrations, from the regular spiral form 

 of the more typical Ammonitidm, may retain a generic value. It is by no means 

 difficult to take the leading genera Ammonites, Turrilites, Baculites, &c, and find 

 trenchant characters ; but it becomes more difficult to find sharp lines of division 

 when we take all the species of the family and endeavor to place each in a well- 

 defined genus. Successive discoveries are every year introducing new links into 

 the chain of genera, until it seems almost impossible to imagine any new form, 

 or any combination of characters on which new genera can be founded. 



In this aberrant group of Crioceras, Helicoceras, Ancyloceras, Heteroceras, Ani- 

 soceras, Toxoceras, and Helicancylus, it seems inevitable that the addition of any 

 other form must almost unavoidably result in the destruction, or coalescing of some 

 of the at present received genera. 



The present genus is founded on a single species, of which I am so fortunate as 

 to possess an almost entire series of fragments of all the parts. Although allied to 

 Pictet's Ani&oceras, it differs in the young shell being a regular descending spiral, 

 and not sinuous. That author figures, in his " Traite de Paleontologie," a very 

 crooked fragment, such as could by no possibility belong to the present shell, any 

 more than it could be a part of a Helicoceras or a Turrilite. 



The figures of various species of Anisoceras, in Pal. Suisse, show a style of orna- 

 ment very similar to the present one. 



In his description, M. Pictet says : " II parait characterise' par une forme plus 

 irreguliere que chez ancune autre e<jphalopode. La coquille dans le jeune age est 

 sinueuse, formant une spirale irreguliere heliciforme, a tours disjoints, ayant tous 

 une double courbure, et ne pouvant pas etre compris dans un plan. Plus tard elle 

 se redresse et s'inflechit en crosse comme les Ancyloceras." It will be seen that 

 the above definition cannot apply to the present genus. 



