340 
NAniLOlDEA. 
those of Nautilus pompilius. He found four layers in each (begin- 
niug with the outside layer) as follows : — 
Aturia Aturi. 
1. External layer of calcareous deposit (= “porous layer” of 
my descriptions). 
2. Nacreous layer, constituting the principal part of the 
neck. 
3. Internal layer of calcareous deposit. 
4. Internal yellow layer (“ couche jaune ’^), applied to the 
previous layer (3). 
Nautilus pompilius. 
1. External calcareous layer. 
2. Principal nacreous layer. 
3. Internal calcareous layer. 
4. Black corneous layer lining the interior. 
Barrande remarks that the first three of these envelopes arc evi- 
dently identical in the two species compared, for the porous layers 
(couches sedimentaires) of Nautilus Aturi cannot be other than the 
porous layers (“ mucoso-cretacees) of Nautilus pompilius, more or 
less changed by chemical reactions after the death of the animal. 
The internal yellow layer (“ couche jaune ”) certainly appears to 
represent by its position the black corneous layer in the interior of 
the siphuncle of Nautilus pompilius. But the difference between 
them is so great that they could not be identified if it were not for 
their exact agreement in position. The difference in their compo- 
tion (the one being calcareous, the other corneous) is not easily 
accounted for ; but it may have been connected with the firmer 
attachment of the animal of Aturia in its shell, as indicated by the 
septal lobes and large invaginated siphuncle contrasted with the 
very slender siphuncle of Nautilus h 
It is further stated by Barrande that the structure and nature of 
the siphuncular elements in Nautilus urnbilicaius appear to be iden- 
tical with those of N. pompilius. 
Einally, one may consider, he says, the siphuncle of the two 
living species of Nautilus {N. pompilius and N. umbilicatus) and 
the siphuncle of Nautilus Aturi, of the Miocene Beds, as presenting, 
fundamentally, the same structure. Barrande draws a parallel be- 
tween the long necks, coexisting with an invaginated siphuncle in 
S.' 
^ See suj)ra, pp. 110-112. 
