8. S. Buchnan — On Jurassic Ammonites. 361 



Arnioceras is an example of a derivative from Cijmhites, acquiring 

 a carina while Cymhites itself remained uncarinate : here the 

 carina becomes of generic importance. But in the cases of sternalis 

 and siibcarinatus there does not seem to be any breaking up of the 

 stock — simply developmental change in Cymbites as a whole. 

 In this case a generic name for sternalis otherwise than that of 

 Cymbites does not appear to be needed ; and Bonarelli's Paroniceras 

 seems not to be required. That author places siibcarinatus again in 

 another genus — Foecilomorphus ; but this is unnecessary for the same 

 reason. Further, the type species of Foecilomorphus is the Am. 

 cycloides, d'Orbigny, which possesses sigmoidal costge, whereas those 

 of subcarinatus are nearly direct. 



Blake and Wright placed siibcarinatus as a Phylloceras, but this 

 was a decidedly unfortunate position, which the septa at once 

 negative. No wonder that Bonarelli in quoting this adds ! after it. 



As to the position in a family arrangement, the inclusion of 

 Cymbites in the Arietidce appears most convenient. The lower forms 

 are decidedly goniatitic in appearance, and the suture-line throughout 

 is of an Arietan character ; further, Cymbites is the radical stock of 

 several series of Arietidce, though it seems also to be the radical 

 of the Polymorphidce,^ Deroceratidce,'^ etc. Biologically considered, 

 however, the Arietidcs are the lowest of the Ammonacece. 



Hyatt did not include sternalis and subcarinatus in his magni- 

 ficent Monograph on the Arietidm ; ^ but he placed Icevigatus 

 in his genus Agassiceras ; in fact, Agassiceras has priority of 

 Cymbites. If it were to be insisted that Agassiceras should be the 

 generic name for the globosus-gvowT^, then Cymbites would be lost, 

 and a new name would have to be found for Scipionianus and allied 

 forms. A reasonable way out of the difficulty seems to be the 

 present use of Cymbites, and the employment of Agassiceras as a 

 generic name with Scipionianus as the type-species. To that genus* 

 would belong Oaudryi (Reynes) Sauzeanus (d'Orbigny) — removed 

 from Coroniceras, where it was placed by Hyatt — and Colesi (J. 

 Buckman), a species which has been altogether overlooked. The 

 only difficulty in the matter is the possibility of Oxynotoceras 

 oxynotum being derived directly from Agassiceras, in which case the 

 name Agassiceras would be rightly employed for the globosns-series, 

 and Oxynotoceras for the Scipionianus-senes, and Cymbites would 

 have to go. 



The present is a suitable opportunity to give a description of 

 Agassiceras Colesi. 



^ Through Foli/morphifes pohjmorphus (Quenstedt) . 



^ Through Microceras plamcostatum (Sowerby). The StephanoceratidtB are 

 descended from the Beroceratidce. The Amaltheidce and Oppelidm are not traceable 

 to any of the Cymbites -iorms directly, but they give ontogenetic evidence of a some- 

 what similar phase in phyletic development. 



^ Smithson. Contrib. Knowledge, 673. 



* To that series latum (" Coroniceras latum,^'' Hyatt) should probably be added as 

 the form connecting Agassiceras with Coroniceras. The readiest generic distinction 

 between Coroniceras and Agassiceras will be found in the greater degeneration of the 

 septa in the latter, Coroniceras proper being distinguished by long lobes. 



