220 GENESIS OF THE ARIETIDjE. 



Oxynoticeras Buvigneri, Hyatt 



Amm. Buvigneri, D'Orb., Tew. Jurass. Ceph., pi. lxxiv. 



Aram. Buvigneri, Dun., Etudes Pal. Bass, du Rhone, p. 147, pi. 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 Grcenoughi 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, Hyatt. 



Plate X. Fig. 23-36. Summ. PI. XIII. Fig. IS. 



Amm. Lotharingus, Keynes, Plates. 



In this species at the size of 100 mm. the keel had almost disappeared, and 

 the pilse 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. This 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, Hyatt. 



Amm. Aballoense, Dum., Etudes Pal. Bass, du Pihone, p. 141, pi. xxvii. fig. 1, 2; pi. xxviii. fig. 1; pi. xxxviii. 

 fig. 1-3; pi. xl. fig. 1. 



This species as described and figured by Dumortier seems to be quite different 

 from Greenovghi, 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. 



