284 BULLIDA. 
draws up the shell to meet the extension, its edges catch and 
raise them to an erect posture. 
The eyes, though not large, are distinct and fixed, in those 
species in which they are not obsolete, as in B. hydatis, and I 
believe in all other Bulle, at the anterior internal bases of the 
so-called tentacular processes; they are not raised, but im- 
mersed in the surface of the skin: though they and the flaps 
or fins are in rather an advanced position, they are only seen 
when the animal is on the march, from its habitude at other 
times of keeping these organs under the shell; but in those 
of thin texture they may be seen through it. The foot, as in 
B. cylindracea, is one entire lobe, divided as in the type at 
the under part from its posterior linear accessory, which also 
hes within the mantle in the narrow aperture of the shell, by 
a deep solution of continuity. In this minute creature it is 
difficult to observe the connection and origin of the accessorial 
lobe with its principal: the deep groove or rather hiatus be- 
tween the two portions of the foot causes the anterior division 
to assume the appearance, as M. Loven terms it, of “solea 
brevissima ;”” the mistake is very natural; the free floating 
margins resulting from its apparent division from the posterior 
lobe, give the appearance of a complete foot. The anterior 
part of the foot is rather shorter than the head-lobe, ovately 
subquadrangular and rounded at the front and posterior ex- 
tremities, sinuated at the centre, grooved around the margin, 
and not at all auricled. I have observed thirteen of these 
animals for four or five days im sea-water, but I never saw 
any reflexion of the mantle; however, the foot at times is 
considerably reflected laterally on itself and the anterior part 
of the shell, and in quietude assumes the quadrilobated 
character of the tribe. The march of this animal is much 
more lively than in B. cylindracea. The mantle forms an 
anal sinus at the posterior part of the canal of the shell; there 
is a deep groove between the head and foot-dises. The bran- 
chial plume is deposited in a cavity across the neck. The 
buccal mass is a light fleshy palate, and may be seen through 
the head-lobe; the mouth leads by a long linear cesophagus 
to the stomach, situate in the last convolute cavity of the 
