1874.] 233 [Hyatt. 



healthy which resembles in every respect the young ofAmaltheus mar- 

 garitatus, having even the crenulated abdomen. This leads into a 

 variety quite common but not so evidently the result of normal or 

 healthy causes. This has a blunter abdomen, which is deeply crenu- 

 lated as in Phylloceras Boblagei, and otherwise resembles it in form 

 though distinct in the septa. The involution, however, is irregular, 

 decreasing with age, instead of preserving the' normal mode of in- 

 crease, though the specimens rarely exceed an inch in diameter. The 

 normal or true oxynotus variety prevails in the majority of specimens, 

 and in these the resemblance of the young to striaries is very much 

 obscured by the early development of the laterally flattened adult 

 form, the sharp keel and specifically characteristic sutures. 



The examination of old and young forms at Semur enables me to 

 state that in extreme old age, when the shell is about 335 mm. in 

 diameter, the form changes. The keel becomes very broad, a de- 

 pressed zone make its appearance on the sides near the umbilicus, and 

 the involution becomes so much less that I have compared the aspect 

 of the umbilicus to that of Amm. Roraani. Here also the structure 

 of the keel in one specimen was plainly visible. Externally the outer 

 shell enveloped the cavity of the keel, internally the nacreous layer 

 formed a convex floor, but the space between, instead of being hollow 

 as in Oxynoticeras GuibaUanum was filled by layers thickest in the 

 centre and gradually fading off to either side, their attenuated lateral 

 extensions forming a third layer between the outer shell and the 

 nacreous lining. The dark colored layer, which is considered as im- 

 portant 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 Lo- 

 tharingus by Reynes 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 involution is perceptibly less, the umbilicus 

 being quite open. Oxynotus is the only species of this group which 

 attains as great a size in its normal variety without losing the keel, 

 and therefore I think this form is also a variety of this species. 

 Amm. oxynotus-numismalis Quenst. (Der Jura, p. 119) is probably a 

 form of this series, and, as supposed by him, is probably identical with 

 Amm. Buvigneri D'Orb. D'Orbigny's original is altered by compres- 

 sion and this defect is represented in his drawing as natural. The 

 original in the Coll. of the Jardin des Plantes has one side more 

 compressed by pressure than the other, showing that the concave zone 



