COLPODASPIS. 31 
In Philine aperta the plantar surface also consists both of foot and 
mantle; but this part of the mantle does not correspond with the 
pallial appendage of Colpodaspis, as it contains the viscera and shell. 
If it be examined, however, from the ventral aspect, the pallial 
siphon is seen on the left hand, as in Colpodaspis (fig. 2), and to the 
right of the siphon, the mantle is seen to be prolonged into a short 
free membranous border, which overhangs the siphonal groove and 
even extends slightly behind it. The relations of this slight expan- 
sion are such that I think it may be regarded as the rudimentary, 
or probably vestigial, representative of the pallial appendage of Co/- 
podaspis. 
'  Radula.—This organ was not described in Sars’ original paper, 
but a figure of it was given (without description) in a later work by 
G. O. Sars (see pl. 9, fig. 9). There is a single admedian series of 
sickle-shaped denticles on either side, and two series of slender later- 
als, the formula thus being 2°1°0:1:2._ I was unable to lay open the 
contracted radula of my specimen, owing to its excessive minute- 
ness; but I determined that the rows in the radula were from 25 to 
30, and isolated individual denticles and half-rows by teasing with 
needle. Some of these are drawn as figure 3 of my Plate. The 
admedian denticles of this radula differ from those figured by Sars 
in presenting a sharp distinction between their terminal and prox- 
imal parts. The handle of the sickle shows an angular projection 
from its inner or concave edge, like the corresponding denticle in 
Colobocephalus costellatus as figured on plate 9, fig.8. The lateral 
denticles also furnish an additional point of resemblance be- 
tween the radule of these two types in that their points are slightly 
bent in a plane at right angles to that of their general surface, so 
that, when the denticles are mounted flat upon a slide, their points 
are directed upwards towards the observer. 
Shell.—Sars has described the shell so accurately that I have noth- 
ing to add to his description ; but my figures being on a larger scale, 
represent its form and wonderful delicacy rather better. 
Summary.—On the whole, I think this Plymouth specimen pre- 
sents features which indicate a slight advance on the organization of 
those described and figured by Sars. I may mention its greater size 
(3:125 mm. as compared with 2°5 mm.), the greater differentation 
of the tentacles, pallial siphon, and admedian denticles, and perhaps 
some increased extension of the free margin of the shell. 
Affinities—Sars was not quite certain whether Colpodaspis be- 
longed to the Opisthobranchia at all, and was much impressed by 
