﻿230 INFERIOR OOLITE AMMONITES. 



1874. Ammonites serrodens, Bumortier. Etudes pal. Bassin Rhone, pi. lviii, 



figs. 2, 3. 

 1885. — — Quenstedt. Amm. Schwabischen Jura, pi. xlviii, 



figs. 15—17. 



Discoidal, compressed, carinate. Whorls sloping from the upper edge of the 

 inner margin to form a sharp, knife-like ventral area. Inner margin fairly steep. 

 Inclusion nearly the whole whorl. Umbilicus small and rather deep. Suture-line 

 simple, the lobes broad with very small denticulations. 



The specimens are very poorly preserved, and lack the test entirely. They are 

 also decidedly inferior as casts — there is no sharpness about them ; they are 

 generally more or less crushed, and a whole whorl is a rarity. On account of 

 these reasons I have contented myself with outline figures, which will convey 

 sufficient idea of the shape. The test was apparently ornamented with fine 

 arcuate growth-lines, but there are only occasional indications of them upon the 

 cast. The smaller umbilicus and the absence — judging from Quenstedt's figure — 

 of ribs in the inner whorls are the features which separate this species from 

 Hudl. affinis. 



From Am. Fredericii, Branco, which, however, is for all practical purposes the 

 same species, it only differs by its less pronounced inner margin. Branco cites the 

 simpler lobes as a character distinguishing his species ; but they do not possess 

 much value, because the lobes of Hudl. serrodens become simpler as the specimens 

 become older or more developed. Dumortier's figure of serrodens (he. cit.) is 

 evidence on this point. 



From Oxynoticeras oxynotum of the Lower Lias, and from Ammonites discus of 

 the Cornbrash, to both of which it bears considerable external resemblance, Hudl. 

 serrodens may be known by its suture-line, as well as by other features. It may 

 possess a certain collateral relationship to the first named, but must be very 

 remote from the latter. The inclusion of these and many other similar- shaped 

 species in the same genus was caused by a failure to observe that they were poly- 

 genetic — that they were, in fact, nothing more than forms resulting from senile 

 degradation of different branches. 



Quenstedt's figures represent Hudl. serrodens without any ribs on the inner 

 whorls. If this be correct, it means that, by very early inheritance of the smooth 

 character of Hudl. Sinon or Hudl. affinis, this species has omitted the ribbed stage 

 altogether. From which of the two above-mentioned species this one is derived 

 cannot be stated with any degree of certainty ; but it is true enough that in Hudl. 

 Sinon, affinis, serrodens, we have, in this order, three stages of development ; and 

 the amount of development is indicated by the ever earlier period at which the 

 smooth, senile character is introduced. 



The upper part of the Jurense-zone (Dumortie7-ia-beds) is the horizon of this 



