Hyatt.] 374 [June 7, 



series to which the individual belongs. This was shown particularly 

 in the macrocephalum series, which continued the direct line of those 

 varieties which began with the true conlractum forms in which the 

 mouth showed little or no contraction. Other series, however, which 

 were followed out from those varieties of Brocchii which did show 

 this contraction, manifested a distinct tendency. It was found that 

 in the same variety the living chamber varied not only in different 

 individuals, but at different stages in the same individual. In the 

 young it showed far less the tendency to contract and to become 

 smooth than it did in the old age of individuals of the same varieties. 

 The contraction and smoothness were also less apparent in the earlier 

 or ancestral forms than in the more mature or descendant forms, 

 whether found in the same formation or in distinct formations. Thus 

 following out from Brocchii to Brongniartii, we find a series steadily 

 decreasing in size, in the regularity of the growth of the shell, and 

 £n the size and prominence of the ribs. The contraction and smooth- 

 ness of the living chamber, at first a variable characteristic, only 

 found in the senile stages of large specimens, become fixed as adult 

 characteristics of all forms in the Gervilii-like varieties of Brocchii, 

 are inherited according to the law of acceleration in the living 

 chambers of Gervilii at an earlier age, and finally constantly appear 

 accompanied by all their attendant degradational or senile charac- 

 teristics at a much earlier period in Steph. Brongniartii. 



The series from Brocchii to microstomum, and also to platystomum, 

 were not worked out in accordance with this theory from "a priori " 

 conclusions, but were traced out in accordance with the evidence, and 

 the true relationship not suspected until these remarks were written; 

 nevertheless the same principles appear to hold in them, but not so 

 well or distinctly marked. 



The microstomum series maintains more determinedly the ancestral 

 Gervilii type so far as the aspect of the ribs is concerned, but obeys 

 the same law in the lateral flattening of the living chamber and 

 increasing smoothness of the species. 



Steph. platystomum is, however, a notable example of the action of 

 the law of acceleration, since here the smoothness and distortion of 

 the living chamber become constant at a very much earlier age than 

 they ever appear in the large Gervilii-like varieties of Brocchii, 

 which, according to Quenstedt's and my own independent observa- 

 tions, must be the immediate progenitor. 



