172 ARTHUR WILLEY. 
symmetrical figure with the outline of the great shell-muscles! 
where the latter abut on the shell. 
In Fig. 17 the union of the marginal pallial artery with the 
left pallio-nuchal artery is represented from the inner surface 
of the mantle. Here it is seen that the two arteries unite in 
the dorso-lateral angle where the mantle and the funnel-flap 
fuse with the body-wall; and furthermore, that from the same 
point an artery is given off which passes forwards and gives 
off branches to the nuchal membrane. The latter structure 
was accurately described by Owen as a “semilunar ridge” 
lying in the concavity at the base of the hood, and applied to 
the involute convexity of the shell. 
Owen thought it might serve to prevent the shell from 
encroaching too much upon the hood ‘‘in the act of creeping.” 
We now know that the animal does not creep on its hood with 
reversed shell. 
The nuchal membrane would seem to be responsible for the 
dense black colour of the involute portion of the shell, and 
possibly also exerts a lubricating influence. Keferstein calls 
it the “ Nackenlappen.” 
(2) Genital Arteries (Fig. 18).—Nolessthan three arteries 
arise directly from the heart which, to my knowledge, have 
hitherto escaped attention. ‘They are (a) the artery of the 
genital duct or gonaducal artery ; (0) the artery of the genital 
gland or genital artery; (c) the artery of the pear-shaped 
gland. 
The accompanying sketch (Fig. 18) obviates the necessity of 
a detailed description. The genital artery is submedian, and 
the main trunk lies on the dorsal side of the genital gland. 
Both the gonaducal artery and the artery of the pear-shaped 
gland give off a branch which passes into the perigonadial 
membrane, and this apparently trifling fact, combined with the 
‘subsymmetrical relations of the gonaduct and the pear-shaped 
gland, may indicate that the latter is the metamorphosed 
‘genital duct of the left side, and not, as I believe has been 
1 For purposes of nomenclature it will be found convenient to speak of the 
great shell-muscles as the columellar muscles. 
