L. F. Spath — Notes on Ammonites. 175 



adaptations, corresponding to elaborately ornamented sphaerocones 

 or uncoiled shells in other lineages. 



In the family Arietidae, the hastening in development of one 

 character of the whorl or its ornamentation faster than another, or 

 the delaying of a feature, afford valuable indications of the beginning 

 of diversity of lineages. During the acme of the group, certain Alpine 

 forms of Coroniceras show a tendency to retard the development of 

 the keel and to flatten the periphery", which tendency probably leads 

 to the Microderocerates. Other Alpine Coroniceras-iorms hasten the 

 development of the keel and foreshadow the somewhat later 

 Aetomoceras, which seems to be a corresponding development of the 

 bituberculate^«ss/cmw. In the later genus, again, the tendency to 

 omit the keel and -to continue the costation across the venter 

 probably leads to Xipheroceras and thus starts thefamily Aegoceratidse, 

 from which the Microderoceras development, mentioned above, 

 would, obviously, have to be excluded. It will be seen that the 

 tendency of a lineage to develop or specialize in a certain direction 

 (adaptative in response to changes of environment or intrinsic and 

 aiming at diversity) is here favoured as the basis of lineal 

 independence and generic classification, as opposed to the morpho- 

 logical method of grouping together forms that fit the generic 

 diagnosis of the textbooks, or to the cyclical method which assumes 

 that stocks should develop according to a given "cycle" and show 

 periods of "anagenesis" and "catagenesis". 



Suess, 1 when first venturing on the subdivision of Ammonites, 

 thought the length of the body-chamber and the shape of the mouth- 

 border characters of systematic value. The former generally depends 

 on shape and coiling, and Buckman and Bather 2 have pointed out that 

 in Stephanoeeras the body-chamber " varies in length from about half 

 a whorl in the thick forms (the supposed females) to very nearly two 

 whorls in the thin forms (the supposed males) ". As a specific 

 distinction this character alone has been used by Pompeckj, 3 who 

 described as Psiloceras Irevicellatum an Ammonite that differs from 

 P. planorhe only in having a shorter body-chamber. On the other 

 hand, G. von Arthaber * bases what appears to the writer to be a very 

 artificial classification of Triassic Ammonites into Microdoma and 

 Macrodoma, on the length of the body-chamber. He also states that 

 " much more important than generally assumed seems to be the con- 

 vergence of forms", but groups such heterochronous homceomorphs 

 as Beloceras and Sageceras together. 



As regards the shape of the mouth-border, great variability is 

 shown even in one genus, e.g., Phylloceras, as H. Douville 5 pointed 

 out. Of course, it might be objected that this genus really ought to 

 be subdivided into a number of genera, but Douville thinks even 

 the ornament of the sides " perhaps more important for classificatory 



1 " Uber Ammoniten " : Sitzungsber. d. Wiener Akad., vol. lii, p. 71, 1865. 



2 Nat. Sci., vol. iv, p. 428, 1894. 



3 " Beitr. z. einer Bevis. d. Amm. d. Schwab. Jura," 1893, pt. i (T). 



4 " Grundziige einer Systematik d. Triad. Amm." : Centralbl. f. Min., etc., 

 1912, p. 245. 



5 " Cerat. de la Craie " : loc. cit., pp. 278-9. 



