XAUTILIDJE. 
339 
author calls it “ la couche de sediment calcaire,” and states that it 
covered the entire surface of the septa, and even the internal wall 
of the shell. I am not able either to verify or to deny this assertion 
from the specimens at my disposal, but there are indications in some 
specimens (seeXo. 34414) of its having been distributed bejmnd the 
limits of the septal necks. At any rate, it lined not only the inte- 
rior of the siphuncle between the nacreous la}^er (lig. 73, b, h) and 
the yellow layer (fig. 73, /, /), but extended for a little distance, 
though in a greatly attenuated state, along the surface of the septal 
necks, where these open out into the chambers (fig. 72, s, s)h In 
Barrande’s ideal section of Aturia Aturi'^ the porous layer of the 
newly formed septal neck is figured, and described as bifurcating 
with the porous layer of the older neck immediately preceding it. 
This, I think, is an error ; I am not sure that they even touch each 
other, though I have represented them as doing so in the figure 
(fig. 73, lower part of figure, to the left of the letter d). ^ It seems 
more than probable that at this point, where the new neck began 
to be formed, there was an interruption of growth, the different 
layers composing it being at their commencement nearly at right 
angles to the older layers (fig. 73, near the part lettered d). 
The porous layer in Ataria is described by Hyatt in his well- 
known paper on the Embryology of Fossil Cephalopods and he 
regards the swollen band as being “ though narrow, the repre- 
sentative of the external sheath of the siphon in Nautilus pom- 
piliusA 
Edwards also, in describing Atiiria ziczac, recognized the porous 
layer, which he described as a “ soft, friable, calcareous sheath, 
which commences near the extremity of the funnel [neck], where it 
touches the preceding funnel, and extends to the end of the preceding 
funnel, to the interior surface of which it forms a sheath.” 
Barrande ® made a careful comparison between the different layers 
composing the siphuncle in Aturia Atari, as above described, and 
^ The general absence of all trace of this porous layer beyond the region of 
the swollen band may be accounted for by removal in the process of fossiliza- 
tion, or by weathering, its extreme tenuity rendering it peculiarly liable to 
destruction. 
2 Syst. Sil. de la Boli^me, vol. ii. pt. i., Atlas, pi. cccclxxix. 
® Bull. Mus. Zool. Cambridge, Mass., 1872, vol. iii. No. 5, p. 95. 
4 Mon. Eocene Mollusca and Univalves of England, vol. i. 1849 (Mon. Pal. 
Soc.), p. 54. 
^ Syst. Sil. de la BohSme, vol. ii. pt. 1 ; texte, pt. iv. 1877, pp. 338-346 ; 
Atlas, pi. cccclxxix. 
z 2 
