BULLA. 275 
the shell ; that part of it covering the branchiz, heart, bladder, 
matrix and testis is of firm texture; the portion within the 
posterior part of the spire, containing the liver, ovarium and 
oviduct, is a mere film, but stronger than would be supposed 
from its tenuity ; at the posterior end of the shell the dorsal 
range is completed by the reflexion on it of the hindermost 
part of the pedal lobe. The anterior part of the under lobe 
forming the foot is precisely of the same length and width as 
the upper one, but more posteriorly it spreads lke a fin, 
which is reflected on the sides of the shell; the disk then 
pursues its course for two-thirds of its length posteriorly, 
at which poimt its continuity is broken by a deep fissure, 
which divides the plate, without injurmg the fabric; on 
the right side near this channel the anus debouches; the 
pedal plate is then continued to the posterior end, becoming 
wider, and there, as before stated, it is reflected on the 
posterior part of the shell. 
It appears then that there are but two lobes, the upper or 
tentacular, and the pedal one, which though interrupted by the 
deep groove, is in reality a single plate, but from its posterior 
and anterior lateral reflexions, together with the tentacular 
disk, gives the animal the characteristic quadrilobated ap- 
pearance of the tribe. The object of the pedal fissure is 
probably to give flexibility to the foot, and allow the lobes to 
act as fins, as the animal is equally an adept in natation and 
reptation. 
Twenty years ago I observed hundreds of these creatures 
swimming and creeping on the fine mud in the lakes of the 
Mount Pleasant Warren near Exmouth; they however sud- 
denly disappeared from the locality, and not one has been 
seen for many years ; the animals now described were obtained 
near Swanage, Dorset. 
The large circumferential canal of the separation of the 
upper and lower lobes is much more decided on the right than 
on the left side, as from the former we have a view of the 
orifice of the verge, the open seminal duct, and the common 
cavity of generation, the testis, and points of the branchie. 
The eyes are very distinct, situate far back on the tentacular 
te 
