278 MR. J. D. MACDONALD ON THE ANATOMY OF NAUTILUS UMBILICATUS 
closely resemble each other as to render it very much easier to trace out their simi- 
larity in corresponding parts, than to determine essential differences between them. 
The specimen of N. umhilicatus examined proved to be a female, a fact which may 
serve to modify the views of those who, adopting the ingenious speculations of 
D’Orbignv with reference to the sexes of the Ammonites as indicated by the cha- 
racters of their shells, apply them also to the several kinds of Nautili known. The 
numbers of the different tentacula in this example, as represented in the following 
table, agree sufficiently well with those of N. Pompilius already reeorded ; consider- 
ing the liability of these organs to exhibit slight modifications in form, arrest of 
development, or supernumerary parts on one or both sides. 
Numerical Table of the Tentacula. 
Left side. 
Right side. 
Total. 
Digital tentacula 
19 
19 
38 
External labial tentacula ... 
12 
12 
24 
Internal labial tentacula ... 
12 
12 
24 
Ocular tentacula 
2 
2 
4 
Total 
45 
45 
90 
As it is not my intention to enter into an elaborate description of the whole ana- 
tomy of N. umhilicatus, seeing that in so doing I should be needlessly repeating all 
those particulars so succinctly detailed by Professor Owen with respeet to the 
N. Pompilius, I therefore purpose making some observations on the mieroscopic 
anatomy of the organs of the speeial senses, and the glandular follicles appended to 
the four vessels which convey the blood from the sinus-system to the branchiae ; and 
in the explanation of the figures, I shall note as they oeeur, all the more important 
matters bearing on the question as to whether the N. umhilicatus is to be regarded 
simply as a variety, of N. Pompilius, or as an originally distinet species. 
0> 'gan of Vision. — To my former observations on the minute anatomy of the eye 
in N. Pompilius I have little more to add, as they are alike applicable to N. umhili- 
catus. I am more fully satisfied than ever that the pigmentary coating is subjacent 
to the retina, and that the filamentous ends of the fusiform eells of the former com- 
mingle with the finely granular and vesicular matter of the latter. I have reason 
to believe also that the long axes of the pigment-cells themselves are perpendicular 
to the surfaee of the retina, like the club-shaped bodies of Jacob’s membrane, and 
parallel examples to this are found in the eyes of some marine Annelidans. 
1 have not been able to trace a vestige of a lens, nor do I believe that such can 
exist ; and the only representative of a vitreous humour is a kind of viscosity which 
appears to protect the retina from the direct action of the sea-water. The exterior 
of the eye (Plate XIV. fig. 1 a) was marked at the back part and in the region of the 
pupil with blotches of the same rich brown pigment which tinted the upper surface 
of the ho(xl and a few of the tentacular sheaths. 
