666 AtE. WALTEE GAESTANG ON COLPODASPIS PTJSILLA. [NoV. 20, 



The foot, upon this interpretation, must accordingly be described 

 as T-square shaped, with gracefully arched anterior wings and 

 rounded extremities, and of about the same length as the shell- 

 bearing portion of the mantle. The median furrow of its plantar 

 surface is shown in my drawing (fig. 2) to have the same extent as 

 in Sars's specimens. 



The Head. — The grooved tentacles in my specimen correspond 

 with Sars's description, except that no mention is made in the 

 latter of a low curved ridge which can be seen in my figure 1 

 crossing the anterior part of the head from side to side and con- 

 necting the postero-dorsal edges of the two tentacles with one 

 another. The eyes also are much closer together in the Plymouth 

 individual than they are represented to be in Sars's figures ; and 

 the statement of the latter that they are situated " close behind 

 and within the base of the tentacles '"' cannot be said to be appli- 

 cable in the present case. I do not, however, think that any great 

 importance should be attached to these slight discrepancies. 



When Colpodasim imsilla is creeping upon a flat surface, the 

 antero-lateral horns of the foot are just perceptibly in advance of 

 the tentacles (fig. 1) ; but when the creature is swimming inverted 

 at the surface of the water the tentacles ^re then seen to be con- 

 siderably in front of the horns of the foot (fig. 2). 



The Body. — I have no addition to make to Sars's account of the 

 body proper, except that in the Plymouth specimen the edges of 

 the pallial siphon were more closely apposed than seems to have 

 been the case with Sars's individuals. 



Pallial ajipendacie. — When the animal is creeping upon the 

 bottom of a vessel, a broad flattened tail-like appendage projects 

 behind the mantle and seems at first sight to be the posterior 

 section of the foot. Examination of the animal from the ventral 

 aspect, however, reveals that this appendage is in reality a pos- 

 terior prolongation of the hinder margin of the mantle to the 

 morphological left of the pallial siphon (fig. 2), 



Sars adduces no homologue of this peculiar appendage, but it is, 

 in my opinion, to be directly compared with the posterior pallial 

 lobes of various genera of Bulloid Tectibranchs. For example, in 

 Haminea cornea Eoule ^ writes as follow'S : — " . . a sa partie j)os- 

 terieure le manteaxi devient assez chamu et forme alors une expansion 

 arrondie que de prime ahord on serait tente de considerer comme 

 Veoctremite du pied. Cette expansion nous parait etre I'analogue du 

 lobe palleal posterieur de droite que Von observe chez le Doridium 

 meckelii, vnais qui serait prive de son Jlagellum." 



Forbes and Hanley ^ mistook this pallial lobe for a " supra-caudal 

 (equivalent to opercular) lobe," but they correctly describe it in 

 Haminea hydatis as being " large and reflected on the spire." 



In Philine catena also, according to Eoule^, the mantle terminates 

 posteriorly in a convex margin, a little below which are two fleshy 



1 L. c. p. 19. 



2 ' British Mollusca,' vol. iii. p. 542, pi. UU. fig. 3. 

 2 l. c. p. 36. 



