1873.] 169 [Hyatt. 



Collenotii has precisely the same form as the old Coroniceras trigona- 

 tum or Asteroceras obiusum, and is also smooth. 



Thus we have the common characteristics of the family produced 

 independently, and in distinct series one after another in regular 

 order in the species of each series, first in the latest member of the 

 Conybeari series, then in the Arnioceras branch, again reproduced in 

 Coroniceras, a descendant branch. Then reduced to a young char- 

 acteristic, and finally abandoned, together with the channel and the 

 form of the whorl and ribs which were also first elaborated in the 

 adults of the Conybeari series, and Arnioceras, and replaced by char- 

 acteristics which have first made their appearance in the old age of 

 these very species themselves. 



Thus the whole group may be compared to an individual taken out 

 of either the highest and latest occurring member of the lower 

 branch, Conybeari, or out of the centre of the Arnioceras branch, in 

 Coroniceras. These have in each individual a smooth stage, or-planor- 

 bis stage, then the adult with keeled, channeled, ribbed, tuberculated 

 stage, followed by an old age in which all of these disappear, and the 

 whorl becomes smooth, the sides convergent, the abdomen narrow and 

 acute. 



The young, therefore, compares with the adult of psilonoius, the 

 adult possesses the characteristics elaborated by successive additions 

 in the growth of the species which intervene between the individual 

 and the point of origin of the series to which it belongs, and the 

 old age points out the changes which must subsequently take place 

 in its own series when the climax of development is reached, and 

 the series is declining. 



In other words, a series of species has, like an individual, a certain 

 store of vital power which enable it for certain periods, more or 

 less prolonged, to evolve new forms and new characteristics, but 

 which in the end fails, and in place of farther progress in that direc- 

 tion we find an evolution of degraded forms, which compare exactly 

 with the retrograde metamorphoses of the individual. 



Size, which indicates vegetative growth, and the power to take in 

 and assimilate large quantities of nutritive matter, which is usually 

 called vital power, corroborates the above. 



The size of the individual increases from psilonoius, which rarely 

 exceeds four or five inches, to Conybeari, which attains the enormous 

 diameter of over three feet. 



Again, in the Arnioceras branch, the first appearing forms are very 



