607 
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 wlrorls in the gerontic stages. 
In Paleozoic shells, like Lurystomites Kellogg’, 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 Anomatloceras anomalum, Trocholitoceras Walcott, 
Lndolobus 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 Mautelus 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, Wohnkammer,”’ p. 259. 
