214 GENESIS OF THE ARIETIDA, 
group occurring in the Kammerkahr Alps. The young being heavily tuber- 
culated and the pile inclined towards the apex, the umbilicus reminds one of 
Ast. oblusum, var. quadragonatum. Vf it should prove identical when the young are 
examined, it would be interesting as showing the early occurrence of this species.’ 
SIXTH, OR OXYNOTICERAN BRANCH. 
OXYNOTICERAS. 
This genus was formerly considered by the author as a distinct family * from 
the Arietide. 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 Arietid, 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 Ammonitine, was at the 
time of our visit at Tiibingen searching for specimens of Oxyn. oaynotum 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 Lotharingum. 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 Lotharingum. 
The young of Oxyn. Greenough’ and G'wibali are very similar in several varieties 
to those of Agas. striaries, and in oxynotum they are almost identical with this spe- 
cies? The young of Lotharingwn, however, have more accelerated development, 
and skip these striaries-like forms, beginning at a very early stage to resemble 
the adult of Greenough. 
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 Agus. 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 Arietide: of the Lower Lias.* 
Oxyn. Greenoughi is in every way very nearly allied to oaynotum, 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 
1 See notes on pages 100 and 205. 2 Proc. Bost. Soc. Nat. Hist., XVII. p. 280, 1874. 
3 See description of Oxyn. orynotum. 
4 Tf, 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. 
