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 Col- 



Radula. This organ was not described in Bars' original paper, 

 but a figure of it was given (without description) in a later work by 

 O. O. Sars (see pi. 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 radulae 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 



