5 
The head still requires some further description. In order to give 
a more correct idea of the mutual superposition of the numerous 
digitations and processes which exist in the Nautilus, instead of the 
eight or ten arms of the dibranchiate Cephalopods*, I have represented 
them from the left side, in three comparative figures, so as they follow 
each other from the exterior surface of the head to the interior cover- 
ing of the mandible (see fig. 5-7). 
In the first place (fig. 5), the mantle f being reflected, the hood 
(a), the different digitations (¢, ¢), and the funnel (d), are visible. 
The large pedunculated and perforated eye (4) has two tentacles 
(ophthalmic tentacles, Owen), one before its anterior margin, the 
other behind, which are however not distinctly seen without reclining 
the surrounding parts, and bending the eye-peduncle*. Only a few 
tentacles are protruded from their sheaths, and partly visible. I never 
saw them protruded to such an extent as in M. Laurillard’s figures. 
The number of these digitations seems not to be exactly the same in 
all specimens. Instead of nineteen digitations on each side, as in 
Prof. Owen’s specimen, I twice found only eighteen. M. Valenciennes 
found only seventeen in his specimen. That the hood is formed ac- 
cording to the ingenious supposition of Prof. Owen, by two large di- 
gitations conjoined along the mesial line, has been mentioned above. 
The hood indeed contains two tentacles, and in this manner the whole 
number of exterior or digital tentacles varies from eighteen to twenty 
on each side. 
The second layer of tentacular processes is brought into view by 
cutting off the hood and the external digitations. Fig. 6 gives a 
view of this dissection. In this figure 4 is the eye, d the funnel, as in 
the foregoing figure ; ¢, ¢ are the cut parts of the tentacles contained 
in the digital processes. The layer now visible is formed by that set 
of tentacular sheaths which Prof. Owen calls the external or superior 
labial processes (fig. 6 4, 4). For a reason explained in the fol- 
lowing part of my paper, I would be disposed to prefer the name of 
external labial process to that of superior. The membrane cover- 
ing the mandibles and the muscular mass of the mouth, and termi- 
nating in the fringed lip encircling those parts, is to be seen at a little 
distance above this layer (at m), and shows numerous circular folds. 
Beneath this layer a small part of the third layer (/) is visible. 
This third layer is brought into view by removing the second (see 
fig. 7). In this figure 4, & are the cut parts of the tentacles of 
the external labial process, and / is the internal or inferior Jabial 
process of the left side. The folded membrane m is now almost 
wholly visible. The internal labial processus consists of a flattened 
stalk, which ascending expands in a compressed paddle, whose supe- 
rior margin is straight and perforated for the exsertion of the tenta- 
cles. There is some likeness to a glove whose fingers are cut off. 
The description of Rumphius mentions all the digitations and pro- 
* Under the eye is a part, first noticed by Valenciennes, a little hollow caruncle, 
with bilabiated aperture, which seems to be the true organ of smell (see fig. 8). 
It is only visible by bending the eye behind and above, and adheres to the root 
of its stalk, 
