﻿G07 



contact it is obvious in all fossils, as it is in the Nautilus, that the 

 outer porcellanous layer is apt to disappear in the contact furrow 

 and that this disappearance is due to contact seems almost beyond 

 question, especially in Schroederoceras and other shells that have 

 free whorls in the gerontic stages. 



In Paleozoic shells, like Eurystomites Kelloggi, Schroederoceras, 

 Estonioceras and many others the loss of the excretory function is 

 only temporary, since the free volution is protected on the dorsum 

 by a thick shell as soon as it begins to depart from the spiral. In 

 all of these that I have observed, the contact area has not been 

 large, but in Anomaloceras anomalum, Trocholitoceras Wakotti, 

 Endolobus avonensis, Tarphyceras and others in which the contact 

 is closer and the furrow broader, the outer porcellanous layer does 

 not pass on to the dorsum. 



Pompeckj* states that the mantle border of Nautilus pompilius on 

 the venter and sides has triple folds and two furrows, which indi- 

 cate that these parts of the rim of the border secreted the outer 

 porcellanous layer which protects the body of the animal on the 

 outer exposed sides. On the dorsum the continuation of this 

 border is entire and not furnished with folds or furrows for secre- 

 tion of the porcellanous layer which is also absent on that side. 



The aperture is not built out on the dorsal side in any involute 

 Nautiloid that I have been able to examine. 



I have not yet been able to find in any of the involute shells 

 observed to have this peculiarity and in which the suppression of 

 the dorsal layer was more complete, that the last volution became 

 free and that the deposition of dorsal shell layers was resumed in. 

 the gerontic stage. The evidence at present from this accords 

 with that to be obtained from coiling, namely, that shells having a 

 certain degree of closeness of contact or involution do not as a rule 

 have a free volution in the gerontic stage. That the aperture 

 might have become free and still be protected by adequate shell 

 layers on the dorsum in the gerontic stage remains to be deter- 

 mined. That this must have been very rare, if it ever occurred, is 

 shown by the fact that no shell has been observed in the Paleozoic 

 and none have been seen in the Mesozoic, Tertiaries or recent 

 Nautiloids, having such a gerontic stage at the apertural end of an 

 involute whorl. 



In recent Nautilus it is especially noticeable, as stated above, 



* Amm. mit " Anormal. Wohnkammcr," p. 259. 



