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that gerontic degeneration is slight and does not affect the amount 

 of involution nor the size of the whorl. This may be due to the 

 rarity of shells that have reached an advanced age or to the brittle- 

 ness of the senile volution, but against this there is sufficient evi- 

 dence. 



Thus, in many Mesozoic fossils and in recent Nautili, shells are 

 often found with the last two or three septa approximating and this 

 is plainly a mark of the failure of the powers of growth and shows 

 in most examples of large size that the animal has probably reached 

 the extreme limits of its existence. 



One fact is of great interest in this connection. Extreme cases 

 of degenerative series are rare among Nautiloidea. The Lituitidae 

 stand alone as the only complete series that can be compared with 

 several that are found among Ammonoidea. The Discoceratidae 

 have also some turbinate genera that can be closely compared with 

 the helicoidal spirals of a number of Ammonitinse. All such forms 

 and others that may be supposed from their characteristics to exhibit 

 similar characteristics, disappear with the Paleozoic and all, so far 

 as I know, before the Carboniferous period. There are phyloge- 

 rontic species like Coloceras globatum in the Carboniferous, but no 

 uncoiled phylogerontic forms. 



In Mesozoic, Tertiary and Cenozoic times, the uniformity of the 

 type is conspicuous, and while it is plainly degenerating from the 

 Carboniferous to the present, this process is not accompanied by 

 the evolution of uncoiled series. The degeneration takes place as 

 stated above in ornamentation of the shell and in the number and 

 variety of the series and forms evolved, but not in the coiling, 

 which is really progressive, nor yet in the sutures, since Aturia is 

 certainly one of the most if not the most highly accelerated and 

 specialized of the whole order. 



These facts all bear directly upon the history of the impressed 

 zone, since in all uncoiled whorls the primitive contact furrow tends 

 to disappear and the outer dorsal porcellanous layer is restored to 

 its full development on that side. 



In Paleozoic time as well as in later times no involute shell has 

 yet been observed with a free gerontic volution, that is to say, when 

 the area of involution reached beyond the limits of the venter and 

 the area covered extended inwardly on to the sides of the next 

 inner whorls, the gerontic stage also remained involute, or, if 

 decreasing in its ventro-dorsal diameters, this decrease never seemed 



