214 GENESIS OF THE AKIETIDiE. 



group occurring in the Kammerkahr Alps. The young being heavily tuber- 

 culated and the pilee inclined towards the apex, the umbilicus reminds one of 

 Ast. oblnsum, var. quadragonatitm. If it should prove identical when the young are 

 examined, it would be interesting as showing the early occurrence of this species. 1 



SIXTH, OR OXYNOTICERAN BRANCH. 



OXYNOTICERAS. 



This genus was formerly considered by the author as a distinct family 2 from 

 the Arietidae. The similarity of the adult sutures, their mode of development, 

 and the affinities of Oxynoticeras with Agassiceras, which it has been possible to 

 trace more fully since the publication of Dumortier's "Etudes Pal. du Bassin du 

 Rhone," show that the genus belongs properly to the Arietidse, notwithstanding 

 the hollow keel. The earlier nealogic stages have the psiloceran form, and the 

 later nealogic stages have the form and characteristics of Agass. striaries, while 

 the adult sutures are unquestionably arietian. 



Baron Schwartz, to whom we are indebted for being made aware of the im- 

 portance of the hollow character of the keel among the Ammonitinse, was at the 

 time of our visit at Tubingen searching for specimens of Oxyn. oxynotum in which 

 the structure of the keel could be studied. Several specimens were subsequently 

 found by the author in the collections at Stuttgardt and Semur, which showed 

 the essentially hollow interior of the keel in oxynotum, and also in Greenoughi, 

 Guibali, and Loiharingum. The late stage of growth at which it appears, and the 

 early senile stage in which it completely disappears, are very marked in Guibali, 

 and especially in Loiharingum. 



The young of Oxyn. Greenoughi and Guibali are very similar in several varieties 

 to those of Agas. striaries, and in oxynotum they are almost identical with this spe- 

 cies. 3 The young of Lotharingum, however, have more accelerated development, 

 and skip these striaries-Wkz forms, beginning at a very early stage to resemble 

 the adult of Greenoughi. 



These facts appear to justify the conclusion that the first appearance of the 

 hollow keel occurred in a genus whose origin is traceable by developmental char- 

 acteristics to the arietian species Agas. striaries, and whatever its subsequent 

 value, whether characteristic of families or common to larger groups, it must be 

 here considered as of generic importance, and certainly not sufficient to outweigh 

 other characters, which bind the oxynoticeran series to their associates among 

 the Arietidae of the Lower Lias. 4 



Oxyn. Greenoughi is in every way very nearly allied to oxynotum, and some vari- 

 eties, especially among the German forms in the Museum of Stuttgardt, have 

 more acute abdomens than the true Greenoughi, approximating very closely to the 



i See notes on pages 100 and 205. 2 Proc. Bost. Soc. Nat. Hist., XVII. p. 230, 1874. 



3 See description of Oxyn. oxynotum. 



4 If, as supposed by Quenstedt, a hollow keel also existed in Agassiceras, this argument is much 

 strengthened, since this genus is in our view more decidedly arietian in its characteristics than Oxynoticeras. 



