21G GENESIS OF THE ARIETID^E. 



Oxyn. Lotharingum, determine this with absolute precision. If this could have 

 been unquestionably settled, the evidence of descent from striaries would have 

 acquired additional probability. ' 



There is also an interesting variety which resembles the young of Amallheus 

 margavitalus, having even the crenulated abdomen, as figured on Plate X. Fig. 19. 

 This leads into a variety having a blunter and deeply crenulated abdomen, as in 

 Phylloceras Boblayi, and also resembling it in form, though distinct in the sutures 

 (Plate X. Fig. 20). The involution, however, is irregular, decreasing with age, 

 instead of preserving the normal amount of increase, though the specimens did 

 not exceed an inch in diameter. These and Quenstedt's observation and figures, 

 especially in his " Ammoniten des Schwabischen Jura," show that all cren- 

 ulated specimens in this group are probably pathological, and also in most cases 

 dwarfish. 1 



The typical form, figured on Plate X. Fig. 18, prevails in the majority of 

 specimens, and the resemblance of these to the young of striaries is very much 

 obscured by the early development of the compressed adult form, the sharp keel, 

 and characteristic sutures of the species. 



The structure of the keel in one specimen at Semur was plainly visible (Plate 

 X. Fig. 27). The external shell enveloped the cavity of the keel and the internal 

 nacreous layer formed a convex floor, but the space between these two, instead of 

 being hollow as in Oxyn. Grecnoughi, was filled by layers of shell. These were 

 thickest at the centre and gradually diminished to either side. Their attenuated 

 lateral extensions formed a third layer between the outer shell and the nacreous 

 lining, but how far this extended upon the sides was not ascertained. The dark- 

 colored layer, which was considered an essential characteristic of a fully de- 

 veloped hollow keel by Baron Schwartz, is also present, lying just above the 

 nacreous lining and a little on one side. 



Besides these forms, there is at Semur, identified as a form of Lotharingus by 

 Keynes, a variety of this species which attains the large size of 393 mm. Even 

 at this size the characteristic form of the adult is maintained, though the involu- 

 tion is perceptibly less, the umbilicus being quite open. Oxyn. oxynotum is the 

 only species of the group which attains as large size in its normal variety without 

 losing the keel, and therefore I think the specimen belongs to this species. The 

 examination of old and young forms at Semur enables us to state, that in extreme 

 old age, when the shell is about 335 mm. in diameter, the form sometimes changes. 

 The keel becomes very broad, a depressed zone makes its appearance on the sides 

 near the umbilicus, and the involution becomes so much less that I have com- 

 pared the aspect of the umbilicus to that of Amm. Romani. 



The examination of specimens of oxynotum in the Ecole des Mines at Paris 

 showed a very thin external layer of shell near the abdomen, a thicker internal 



1 The small specimens figured by Canavari on Plate VI. of the work so often quoted above, as Amaltheus 

 margarilatus and actmonoides, and Ariet. (Oxynoticeras) Castagnolai, are probably all related to these peculiar 

 pathological forms, and are, notwithstanding their close imitation of the characteristics of Amaltheus, really 

 only morphological equivalents. Canavari has himself referred Castagnolai to Oxynoticeras, and this is 

 evidently an entirely distinct species from that figured by Wahner as Castagnolai, which in our opinion is a 

 species of Caloceras. 



