1876.] 381 [Hyatt. 



none at all. All of these exceptions, however, can be accounted for 

 by natural causes, and the comparison between the life of the group 

 and the life of the individual is rendered even closer and more dis- 

 tinct thereby. I have frequently, in former publications, referred 

 to these facts, and am interested in them now only in their applica- 

 tion to the present group. 



We find in looking at the table (p. 366) that all the series sprang 

 from one ancestral form, and that as in many other cases among Am- 

 monoids, the genesis of the forms must have proceeded Avith compara- 

 tive rapidity. This of course means with reference not to the number 

 of years, but to the portion of geological time occupied by a series. 

 Thus the whole of the time during which the Oolites were being 

 deposited, was not needed in order to produce the extreme forms of 

 the Sauzel group by evolution out of nodosum; on the contrary, one 

 single bed contains the entire record of their existence, one minor 

 period alone was amply sufficient for the evolution of the most 

 aberrant form of the whole genus. 



If we assume that certain characteristics which show themselves 

 for the first time in the organization of Steph. Humphriesianum, 

 subcoronatum, contractum, etc., were favorable to these forms, and 

 particularly fitted them to sustain existence in these different local- 

 ities and with distinct physical surroundings; and that these different 

 characteristics were directly due to the necessity of the plastic 

 organization to flow into and fill up certain vacancies, and fit itself 

 to fill these vacancies more and more completely, we can under- 

 stand how the differences which distinguish the forms have arisen. 

 Thus the peculiar lappets of the rim of the mouth in Steph. Sauzei, 

 and the numerous local peculiarities of appearing here and there in 

 the history of every fauna, which are merely varietal and not char- 

 acteristic of the series or even of the species, could be accounted 

 for. They are characteristics which suddenly appear without having 

 had existence previously in ancestral forms. 



Besides these, however, there are numerous other characteristics, 

 those which are derived from ancestral forms and are mostly con- 

 fined to the young, such as the Pettos-form and characteristics. 

 These are permanent and hereditary, and apparently independent 

 of the surroundings in proportion to the antiquity of their source. 

 Thus the extreme bag-like embryo is invariably present, and there is 

 every intermediate grade from this to the full Pcttos-like form and 

 tubercles, etc., which last, on account of their recent origin, are, ac- 



