xii YORKSHIRE TYPE AMMONITES Dec. 



in part. Then Criocone is from (cpidr, the Ionic volute, which gave 

 Crioceras ; and the rest are similarly obvious. 



Further, there will be use for Turricone instead of Hyatt's 

 Turriliticone : it is really a sphaerocone developed as a turreted spiral ; 

 for Scaphiticone, from Scaphites, a sphaerocone finishing with an 

 attempt towards a criocone ; and Oxycone, for highly-compressed 

 forms which are, more or less, in the occlusal stage, but are too thin 

 to be sphaerocones — Am. oxynotus is an example. 



Practically all the Nautiloids and Ammonoids may be distributed 

 among these cone-stages. A species may show several stages in its 

 ontogeny — especially in higher (later) forms ; and its development may 

 not be continuously forward in the direction of the arrow, but may be 

 backward, especially in gerontic stages. 



For instance, the Nautiloids show the pre-occlusal and occlusal 

 stages, but they have developed nothing later than the sphaerocone 

 stage. Lituites, however, which attains to the ophiocone stage, does 

 not develop a sphaerocone, but runs back to produce an orthocone. 



Among Ammonoids the pre-occlusal stages are shown properly by 

 few genera, Badrites, Mimoceras, for instance. In most cases the pre- 

 occlusal stages are highly condensed, so that Jurassic Ammonitoids, 

 for instance, practically begin as sphaerocones ; they then develop to 

 become serpenticones, and may again pass to be sphaerocones, making 

 a subsidiary cycle (see p. xiv ) ; or after the serpenticone stage they 

 may develop as oxycones — a similar cycle, except for compression, 

 for both cycles are from occlusal to serpental, to occlusal again. After 

 the oxycone stage there is often sudden umbilical expansion, a sort 

 of incipient scaphiticone stage : many Hildoceratidae are oxycones 

 ending as incipient scaphiticones. Among some of the so-called scaphitoid 

 Ammonites this is very marked, Creniceras, Cadomoceras ; but other 

 scaphitoids are sphaerocones ending as incipient scaphiticones, Siiinena. 



Thus, while the pre-occlusal and occlusal stages are exhibited by 

 Nautiloids, the bulk of the Ammonoids show the occlusal and post- 

 occlusal stages ; and the Ammonitoids show a sphterocone radical 

 developing post-occlusal stages, with subsidiary cycles. 



Thus the Nautiloids show an incomplete cycle — the half up to the 

 occlusal only ; but the Ammonoids exhibit all the stages of the complete 

 cycle. Among Ammonitoids there is great mortality in the post- 

 serpenticone stages, especially in the oxycone. Several stocks also appear 

 to finish as criocones, or toxocones ; and perhaps only one or two stocks 

 can be regarded as attaining to the baculicone stage. 



With the above terms it will be possible to state concisely the stage 

 of evolution of the conch in the species to be described ; and the position 

 of any species in regard to the cycle of cone-development may be readily 

 understood. 



These terms can also be applied to other molluscs. Thus, Tentaculites 

 are orthocones. Among Gastropods, Bellerophon is a sphaerocone. In 

 the Capulidae are cyrtocones and turricones ; but some which look like 

 cyrtocones are possibly toxocones : that is, they are, perhaps, post- 

 turriconic. The Vermetidae show turricones trying to become baculi- 

 cones ; but the bulk of Gastropods are turricones, showing a kind of 

 subsidiary cycle within the turriconic development, from occlusal to 

 serpental, to occlusal again. 



