﻿226 INFERIOR OOLITE AMMONITES. 



The descent of the genus Hudlestonia is very uncertain, because of the paucity 

 of the badly-preserved material which I have had to examine ; but the general 

 shape and the suture-line suggest affinity with Oxynoticeras. Now, Hyatt derives 

 Oxynoticeras directly from Agassiceras striaries, and places the genus in the family 

 Arietidse. 1 I am almost inclined to think that there is an intermediate stage of 

 development, of which all trace is omitted in Oxynoticeras on account of the very 

 early inheritance of Oxynoticeratan characters ; and this stage may be mentioned as 

 Amm. obtusus — stellaris — impendens — Gollenotii, Wright — Gollenotii, d'Orbigny 

 (Slmpsoni, Wright) — oxynotus. The changes in this series are increase of involu- 

 tion and consequent broadening of lateral area — decrease of ribbing — compression 

 of ventral area, with consequent gradual loss of ventral furrows, ending in an 

 a^ute carina. 



The development of the genus Hudlestonia seems to be almost parallel 

 to this, and appears to furnish strong evidence in support of the above view con- 

 cerning Oxynoticeras. Hudl. Sinon may be taken as analogous to Am. Gollenotii, 

 Wright, in the matter of its ribbing, &c. ; but Hudl. serrodens, which is analogous 

 to Oxynot. oxynotum, shows no trace of this ribbing, &c., 2 in its inner whorls — in 

 fact, the Sinon-st&go is omitted in Hudl. serrodens on account of the very early 

 inheritance of the smooth characters. The inner whorls of Hudl. affinis (PL 

 XXXVIII) show the connection of Hudl. serrodens with Hudl. Sivion, and indicate 

 that Hudlestonia was derived from an Ammonite bearing considerable resemblance 

 to Arnioceras except in lacking furrows ; and the absence of furrows may of course 

 be due to development. 



Further than this it is impossible to go with the poor material at command ; 

 but I think there is sufficient evidence to warrant the conclusion that Hudlestonia 

 is a developed form of a branch of the Arietidse, say Arnioceras, and is analogous 

 to Oxynoticeras — in other words, that it is a later development from the same or 

 nearly similar stock whence Oxynoticeras was derived, and that it went through 

 very similar phases of development with similar results. The young forms of the 

 genus show by their ribbed whorls and, comparatively-speaking, large umbilicus 

 that Hudlestoyiia is not descended directly from Oxynoticeras ; and the umbilicus, 

 again, prohibits the idea that they come from Am. Guibali, whose ancestors, by the 



that practically bis genus Pelecoceras and my genus Haugia are identical, he kindly wrote in reply, 

 " I noticed that you had separated the Variabilis-gvoup from Hammatoceras by name of Haugia. . 

 . . . It is curious that while I separated Pelecoceras I did not separate Variabilis itself from 

 Hammatoceras. I do not think, therefore, that my name Pelecoceras has any proper title to super- 

 sede your Haugia." I take this opportunity to tender my sincere thanks to the learned Professor 

 for his constant courtesy. 



1 " Genesis of the Arietidse," ' Smithsonian Contributions to Knowledge ' (4to.), vol. xxvi, p. 214, 

 ISSi). 



2 That is according to Quenstedt's figure. I can say nothing from my own specimens. 



