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Vestinautiius Konincki leads into such forms as Vestinautilus pin- 

 guis, Figs. 16-19, which has the ridged characters, etc., confined 

 to the nepionic stage, which is somewhat abbreviated. The sub- 

 spinous characteristics are also crowded back and replaced earlier 

 by gerontic modifications similar to those which occur only in the 

 senile stage of V. Konincki. Thus these degenerative changes are 

 shown to occur in what is properly the parephebic substage of V. 

 pinguis. The history of the impressed zone accords with that of 

 the other characters and may be seen in the figures to have been 

 introduced as an acquired character dependent upon close coiling. 

 It is not present in Triboloceras nor in the nepionic or ananeanic 

 substages of the nautilian forms. It appears only after contact, and 

 in other words is a contact furrow and its characteristics are deter- 

 mined wholly by the moulding of the dorsum on the peculiar ven- 

 tral surfaces which are encountered during growth. 



COLOCERAS GLOBATUM. 



Nautilus globatus, De Koninck (Calc. Carbon., PI. xxxi). 



PI. x, Figs. 1-14. 



The development of this species was partially described in my 

 "Carboniferous Cephalopods," second paper, Fourth Aim. Rept. 

 Geol. Surv. Texas, p. 447-451, but no figures were given and the 

 genus Coloceras was then erroneously referred to the same genetic 

 series as Coelogasteroceras. More extended study of both of these 

 forms has shown me that the latter belongs to a distinct series. 

 Coloceras globatum has the peculiar lateral flutes and characteristics 

 of the Triboloceratidae in the nepionic and neanic stages, and the 

 hollow ventral zone of the paranepionic substage, which led me to 

 suppose that it belonged to the same genetic series as Coelogastero- 

 ceras, may be accounted for equally well when C globatum is referred 

 to the Triboloceratidae. Figs. 5 and 6, PI. ix, of Vestinautilus Kon- 

 incki show that the broad, hollow, ventral zone of the ephebic > 

 stage becomes narrow and the abdomen is gibbous on either side of 

 it in the anagerontic substage of this form. 



The similarity of the ventral hollow zone of the young of C. 

 globatum may be accounted for, if it is supposed to be ah accele- 

 rated phylogerontic character. The only difficulty in the way of 

 this assumption is the preexistence of the lateral flutes in the neanic 

 stage. I have, however, frequently seen similar examples of the 

 unequal acceleration of characters and this is probably another of 



