Hyatt.] 398 [December 20, 



iatites there is a repetition of the characteristics of the Nautilini in 

 the second septum and the pure Goniatite does not appear until the 

 third septum. 



Among the Nautiloids the same formation of the siphon is found, 

 but the coecum is entirely out of the embryonic ovi- shell, which has 

 left the coecum entirely isolated inside of the apex of the first whorl. 

 The first septum of the recent Nautilus has a faint ventral cell, is 

 very shallow, not parallel with the area of cicatrix, and is penetrated 

 by a dorsal lobe. 



The first septum of Nautilus atratus of the Jura is very shallow, 

 nearly parallel with the area of the scar, 1 and not penetrated by a dor- 

 sal lobe. In outline, suture and position of siphon, it resembles the 

 form of Nautilus Bohemicus of the Silurian, until it attains a much 

 older period. The second septum has the dorsal lobe and is other- 

 wise changed, and this is the representative of the first septum in 

 Nautilus pompilius. 



The first septum of Nautilus Koninckii of the Carboniferous differs 

 but little from the succeeding septa, except in being shallower. It is 

 very shallow, and has nearly the same outlines as the first septum of 

 Nautilus atratus or lineatus. The septa of Nautilus Bohemicus, until 

 a much later period, possess apparently similar shallow ventral and 

 abdominal cells, though in the adult these become too deep for com- 

 parison. The evidence is sufficient, however, to show that the first 

 ten or twelve septa of Nautilus Bohemicus of the Silurian, and the 

 earlier septa of Nautilus Koninckii of the Carboniferous, are all repre- 

 sented by one stage of growth in one septum in Nautilus atratus of the 

 Jura ; then that this stage is skipped, or left out entirely in Nautilus 

 pompilius of the present epoch. This is the law of acceleration, or the 

 perpetual reduction of adult characteristics to earlier and earlier peri- 

 ods in the growth of the later existing individuals, until finally many 

 characteristics altogether disappear. The shell of Ammonites, Gon- 

 iatites, and Nautilus is composed of two layers, the external layer 

 being separable into two strata, an outer colored stratum and an inner 

 whiter stratum. All of these are imbricated so as to show that they 

 are undoubtedly made up of zones of growth laid on from within by 

 the edge of the mantle. 



1 This scar is the real apex of the whorl, not the apparent apex, but lies on the 

 dorsal side of the angular termination of the whorl. 



