1903. | POLYCLADS OF THE ‘‘ SKEAT EXPEDITION.” 317 
racters of the genital organs, but, owing to the condition of the 
specimen, it is obviously far from being satisfactory, and in order 
to deal fully with this interesting species more material is 
necessary. 
PROSTHIOSTOMUM PALLIDUM, sp. 0. 
One specimen, from the sea-shore, Dec. 1899. 
Total length, about ......... 20 mm. 
Breadth, about ..............- 4 
Anterior margin rounded. Arrangement of eye-spots shown 
in text-fig. 56. 
9 
Text-fig. 56. 
br. e. 
Eye-spots of Prosthiostomum pallidum. 
e, marginal; b7.e, brain eye-spots. 
Closely allied to P. siphunculus of the Mediterranean by the 
arrangement of the eye-spots, and agreeing with it apparently in 
being, so far as the spirit-specimen shows, of an uninterrupted 
dull grey-brown colour, it differs sutticiently in that the two rows 
of brain-eyes diverge continuously from one another from before 
backwards, whilst in the Mediterranean species these two rows 
converge at their middle. 
It is also readily distinguishable, I think, from the latter species 
by its smaller size. The single specimen obtained by Mr. Evans is, 
to judge from the state of the sexual apparatus, fully mature. 
P. siphunculus, moreover, has the genital sexual organs relatively 
much smaller, to judge from Lang’s figures (see Lang, ‘ Poly- 
cladida,’ pl. 5. fig. 3). 
P. pallidum is certainly very distinct from any species from 
the Indian Ocean that I have had the opportunity of studying, 
and also, I believe, from the species found in the Pacific. 
I believ e that Leptoplana aurantiaca ot Collingwood is really a 
Prosthiostomum. It has the shape and proportions of a member 
of that genus, whilst its eye-spots have the characteristic Pros- 
thiostomum arrangement. It may, of course, be identical with the 
