PBOSOBRANCH GENUS PONTIOTHAUMA. 461 
septum of P. mirabile being represented by an encircling fold 
which is probably quite incapable of sufficient extension as to 
form a complete septum. The posterior chamber appears com- 
paratively short; and the introvert in the retracted condition 
extends well into the anterior chamber. ‘The introvert is a 
perfectly conical structure with but a minute terminal aperture; 
its interior is not spongy as it is in the type-species. 
The alimentary canal (Pl. 42. fig. 11) has the characteristic 
simple U-shape and is of wide calibre throughout its course. 
The pharyngeal bulb is ovoidal, with thick muscular walls, and 
opening into the proboscis by a large circular opening. Imme- 
diately posterior to the pharynx the cesophagus, where it is 
encircled by the nerve-ring, becomes constricted and then again 
enlarges ; but the constriction is not nearly so sharp as it is in 
P. mirabile. The post-neural enlargement being most marked 
dorsally (Pl. 42. fig. 18, Oe.), the gut in this region appears 
laterally compressed. The lumen of this segment of the esophagus 
is almost occluded by the enormous development of the longitu- 
dinal plication of its inner wall. | 
The radula-sac (R.S.) is minute; its duct enters the floor of 
the pharyngeal bulb (B.M.) at about its middle and considerably 
to the right side. The teeth (PI. 42. fig. 14) of the radula are 
similar to those of P. mirabile ; but they are shorter and stouter 
in proportion, and the terminal barb is represented by but a 
slight thickening. 
The salivary glands (S8.GI.) are disposed one on either side of 
the radula-sac, into which their ducts open. . They are somewhat 
retort-shaped structures ; their elongated, sharply recurved, distal 
prolongations appear to pass into the body-wall. 
The poison-gland (L.G1.) is of the normal type; but, as in 
P. mirabile, its duct, which is very long and much convoluted, 
enters the pharyngeal bulb ventrally at a point a little behind 
and somewhat to the right of the radula-sac. 
The nervous system being injured, no attempt has been made 
to work out its detailed arrangement. It is noteworthy that, as 
in P. mirabile, the pedal ganglia lie to the right side, and that, in 
addition, the cerebral ganglia are much displaced so as to he 
aimost below the cesophagus to the left side. Contrary to the 
condition met with in P. mérabile, the cerebral and pedal ganglia 
are separated by but short, stout connectives. 
