BULLA. 283 
disc, mentioned in B. hydatis, with the groove that is the con- 
tinuation of the vas deferens; and such of the other organs 
that could be observed in so minute a species scarcely differ 
from the type. 
M. Loven mentions eyes imbedded in the skin; no appli- 
ances enabled us to detect them in this species; but whether 
they are there or obsolete, that circumstance is only specific : 
ex. gr. in Natica monilifera the eyes are obsolete, but in 
N. Alderi they are easily seen; the presence or absence of 
these organs is therefore not of generic value. All the other 
points beg absolutely similar to the type, we are bound to 
consider the genus Cylichna as not applicable to B. cylindracea. 
We have a preparation to show that the linear posterior lobe 
of the foot lies in the narrow aperture of the shell, agreeably 
to the variable characters of the Bulle. 
B. truncata, Montagu et Auct. et nobis. 
Cylichna truncata, Loven. 
C. truncata, Brit. Moll. ui. p. 510, pl. 114. B. f. 7,8; (animal) PvE 
f. 4. 
Animal subcylindrical, convolute, flake-white ; mantle hya- 
line, quite plain, extending occasionally beyond the margins 
of the shell. 'The head lobe is large, compressed, very slightly 
auricled, sinuated in the centre, and edged with a light lead- 
coloured line, which may be the reflexion through the hyaline 
tissue of the leaflets within the anterior part of the sinus 
between the lobes, mentioned as conspicuous in B. hydatis, 
and presumed to be seen in the last species. 
The mouth is between the head lobe and that of the foot. 
The animal carries the head rather beyond the foot on which 
it lies, so that it may be mistaken for that organ. The pos- 
terior division of the head-dise from the neck, with the deep 
central sinuations, give rise to what I call the flaps or fins, 
which in this animal are more extensive than in B. cylindracea, 
and have the shape of short, flat, triangular pointed fillets, 
lyig like the ears of a hare close to each side of the neck ; 
but when the animal im marching, after having fixed the foot, 
