220 GENESIS OF THE ARIETID. 
Oxynoticeras Buvigneri, Hyarr 
Amm. Buvigneri, D’OrB., Terr. Jurass. Ceph., pl. Ixxiv. 
Amm., Buvigneri, Dum., Etudes Pal. Bass. du Rhone, p. 147, pl. xxxiv. 
The original is not correctly figured by D’Orbigny. The specimen is altered 
by compression, and this distortion is represented in his figure as natural; it has 
one side more compressed than the other, and this side has been selected in his 
figure as characteristic of both sides. The abdomen of the original also possesses 
a keel, which is not shown in the figure. The figure is, however, near enough 
to that given by Dumortier to enable one to identify it as the same, and the fact 
that it has a keel is an important point in this connection. It is much more 
involute than any species of the Gcenoughi subseries except Lotharingum, but 
from this last it can be distinguished by the much larger size attained before 
the keel is lost. Dumortier’s specimen reached the diameter of 126 mm., and 
D’Orbigny’s that of 184 mm., without perceptible marks of senile degeneration. 
We regard this difference as an uncertain characteristic, but have no means of, 
verifying the connection with Lotharingum. 
Oxynoticeras Lotharingum, Hyarr. 
Plate X. Fig. 23-26. Summ. Pl. XIII. Fig. 15. 
Amm. Lotharingus, Reynés, Plates. 
In this species at the size of 100 mm. the keel had almost disappeared, and 
the pile in several instances crossed the abdomen, The abdomen had become 
rounded, but the involution had not perceptibly decreased. The umbilicus is 
smaller in the adult, the whorls stouter in proportion than in the preceding spe- 
cies, and the characteristic form and aspect of Greenoughi are found only in the 
young. The younger stages had a solid keel, the hollow keel occurring only 
at later stages of growth and in adults, and it suffered from degeneration and 
finally disappeared in the senile stage. ‘his is one of the most interesting ex- 
amples yet discovered of the similarities of the old and young stages in the same 
individual. The resemblances which usually exist between the old and young 
shell are also present, and the absence of the hollow keel in extreme old age 
shows how seriously the organization may degenerate after the adult period, 
even with regard to the most important structural differentiations, 
Oxynoticeras Aballoense, Hyarr.: 
Amm, Aballoense, Dum., Etudes Pal. Bass. du Rhone, p. 141, pl. xxvii. fig. 1, 2; pl. xxviii. fig. 1; pl. xxxviii. 
fig. 1-3; pl. xl. fig. 1. 
This species as described and figured by Dumortier seems to be quite different 
from Greenoughi, and yet the stouter specimens of that species certainly approxi- 
mate to it quite closely. We have not the means at hand of finding by com- 
parison whether the principal characteristics cited by Dumortier, namely, the 
deep umbilicus and abrupt shoulders of the whorls on the edge of the umbilici, 
