172 L. F. Spath — Notes on Ammonites. 



with the Cretaceous Ancyloceras or Toxoceras, as, e.g., d'Orbigny and 

 Pictet did, simply because they are of a similar shape. There 

 is no generic connexion among most of the scaphitoid or other 

 "aberrant" forms that appear at a number of horizons and may 

 originate from very distant stocks, though they have a similar form. 

 De Loriol 1 describes an aberrant form that occurs together with 

 Cardioceras cor datum as (Ecopty chins Christoli, Beaudouin. It is 

 evident that this form has no connexion with the earlier type of 

 CEcoptychius, namely CE. refracius (de Haan), from the anceps zone. 

 There is not even similarity of shape, and the form is probably a 

 modified development of some contemporaneous group of Ammonites 

 such as Pachyceras. It has also been mentioned already that it is 

 impossible to group together the Devonian Biloceras and the Triassic 

 Sageceras, or the Cretaceous Garnieria and the Liassic Oxynoticeras, 

 simply because they are similar in appearance. It will be noticed 

 that geological occurrence is a determining factor in the separation of 

 many of these lineages based on a modified whorl-shape ; Spiroceras 

 is strictly Callovian, Ancyloceras confined to the Lower Aptian. 

 Unfortunately this has given rise to a multitude of new names, but 

 the creation of separate genera for the various abnormal whorl-shapes 

 antedates the splitting up of the genus "Ammonites" by Suess, Hyatt, 

 and Waagen . 



Impossible as it may be to use form and coiling of the shell for a 

 general classification, certain Ammonites (e.g., Phylloceras) can at 

 once be recognized by their form, and when this is modified (e.g., 

 in Soiverbyceras) a separate name is given to the new stock. The 

 same thing applies to the coiling in Lytoceras where evolution gives 

 rise to, e.g., Costidiscus and Macroscaphites. In the great majority of 

 Ammonites, of course, form and coiling vary considerably within 

 a genus; e.g., in Cadoceras there are compressed shells and greatly 

 depressed cadicones, in Morphoceras there is involution and evolution, 

 in Schlotheimia the whorl may become almost oxynote (S. Greenoughi). 

 When form and coiling change within a species group, what older 

 authors termed thick and thin, e volute or involute " varieties " of the 

 species are produced. But the term " variety ", which has a definite 

 zoological meaning, is not favoured by modern palaeontologists, who, 

 on the other hand, often do not make enough allowance for individual 

 valuation within a species. It may be inadvisable to give a new 

 name to every form that differs, often only very slightly, from the 

 type in thickness or involution ; but it seems to the writer equally 

 objectionable to identify, e.g., a Yorkshire Psiloceras with an Alpine 

 form only because their dimensions agree (for in both the erugatum 

 and the calliphyllum species-groups similar variations would probably 

 have occurred), or, as Hyatt has done, identify an Amioceras from 

 Peru with A. ceras, Giebel, sp., when there is such a distinct change 

 in the Amioceras fauna even from Gloucestershire to Dorset on the 

 one hand and to Yorkshire on the other. It is known from the 

 study of living mollusca that in the sea each locality gives its 



1 "Etude sur les Mollusques et Brachiopodes de l'Oxfordien Super, et 

 Moyen du Jura Bernois," Supplement I : Mem. Soe. Pal. Suisse, vol. xxviii, 

 pp. 20-2, 1901. 



