Hyatt.] 



22 [October 5, 



some months before they had found a large bone there about 

 eio-hteen inches long which had been taken by some professor of the 

 University. 



Mr. Bouve thought this object was a natural production, in 

 which opinion other members who examined it concurred. 



On Reversions among the Ammonites. By Prof. A. Hyatt. 



In some remarks relating to the origin of characteristics among 

 animals, Mr. Hyatt stated that he had recently discovered a series 

 of reversionary characteristics among the Ammonites which might 

 be considered worthy of exceptional consideration. 



These are the peculiar extensions of the pilae (ribs so called) and 

 of the intervening sulci, or lateral depressions, across the abdomen 

 or external periphery of the shell, characteristics found especially in 

 Microceras planlcosta and laticosta. The genus which was founded 

 upon this peculiarity and the (" Discoceratidse)" Arietidse, having 

 been recently subjected to a revision, certain similarities of a very 

 remarkable kind were observed. 



It was found, that among the Arietidae, Coroniceras rotiforme, 

 occurring in the " Bucklandibett " of Oppel, Cor. nodosum, in the 

 upper part of the same bed, Amm. Birchii, just above this in the 

 " Tuberculatusbett " and Asteroceras obtusum, still later in the "Obtu- 

 susbett," all exhibited to a greater or less degree the planicostan pilse 

 on the abdomen during some stage of growth in certain individuals. 



In Coroniceras nodosum this is especially remarkable, and the con- 

 trast between the young in those individuals which show this stage, 

 and the adults, with keel channels and septa all so typically arie- 

 tian in character, is very great. In all these species the planicos- 

 tan stage appears only in a limited number of individuals in each 

 species, and is always succeeded in course of growth by the features 

 just described of keel, channels and septa, peculiar to the family of 

 the Arietidae. In Opliioceras raricostatum, however, the latest 

 occurring species of the lower Lias which has the typical septa of this 

 family, the planicostan stage is superseded in course of growth only 

 by a keel, this species having no channels. 



Of course, in trying to account for the presence of this transient 

 characteristic, one follows the family back to its lowest representa- 

 tives. These may be said to be two species, Caloceras torus and 

 Arnioceras cuneiforme, the former closely allied to Psiloceras psilo^ 



