1874.] 17 [Hyatt. 



JEgoceras Charmassei and Leigneletii, and according to Oppel, all 

 these forms are in the "Angulatusbett," succeeded in the " Tubercu- 

 latusbett," by JEgoceras Boucaultianum. If there is really any such 

 regularity of succession, and from the collection at Semur it Vould 

 seem to be even more regular than Oppel supposed, it would accord 

 admirably with what has been observed in other groups. 



Not only does the involution greatly increase in each succeeding 

 species, but the septa become more complicated in outline, and the 

 adult characteristics of the pilse 1 and form are repeated at earlier and 

 earlier stages in each species. This may be seen by the following 

 descriptions. The same law governs also the inheritance of the old 

 age characteristics of the individual. Thus Boucaultianus has the 

 old age characteristics sooner developed in its growth than any other 

 form, and occurs latest in time, thus showing that the acceleration, or 

 quicker reproduction of the characteristics, extends to the whole life, 

 affecting even the period at which old age begins. The size increases 

 in each successive form to Leigneletii, and then decreases considerably 

 in Boucaultianus. 



One specimen from Semur is labelled Ammonites Boucaultianus but 

 evidently belongs to Leigneletii. This shows that in extreme old ao-e 

 the abdomen becomes perfectly sharp and smooth; the pilse are 

 obsolescing, not reaching quite to the edge of the abdomen. 



In Prof. Fraas' collection, associated with P. planorbis in the 

 Planorbisbed, is a specimen of JEgoceras angulatum var. catenatum 

 and as- this is the first appearance of JEgoceras angulatum, it is in- 

 teresting to notice that it is less involute, more discoidal, and the 

 whorl is more involute in aspect, or more like P. planorbis in its 

 proportions than the members of the same group, which follow in the 

 Angulatusbed. 



It seems to me, therefore, that both by its geological position and 

 characteristics it deserves to retain the separate appellation of JEgo- 

 ceras catenatum. The developmental histories of both catenatus and 

 angulatus, seem at first sight to contradict the supposition that they 

 can be traced to P. planorbis, since the resemblances of the adults 

 disappear and the differences become more and more prominent as 

 the shells are traced backward to their younger stages of growth. 



In the collection at Semur there are three specimens in the Planor- 

 bisbed under the name of catenatus. They are not large, but one 

 exhibits obsolescing ribs and a smooth abdomen at the diameter of 

 1 Pilas is used as synonymous with ribs. 



PROCEEDINGS B. S. N. H. — VOL. XVII. 2 OCTOBEB, 1874. 



