404 On a Group of the Aplysiidai. 



collection), and by a specimen in a good state of preservation 

 from Siam (M. Mouliot's collection). 



This latter (PI. XVIII. figs. 1 and 3) appears to be a new- 

 species, which I am enabled to describe Idj permission of 

 Dr. Glinthcr, Keeper of the Zoological Department, and which 

 may be designated Aplysia Mouhoti. It closely resembles 

 A. piperata (figs. 2 and 4) in the general structure of the body 

 and in colouring. It is, however, well differentiated from it : 



(1) by the absence of the prolongation of the mantle into a 

 long excretory siphon posteriorly (figs. 3 and 4, siph.). (2) The 

 pleuropodia also are somewhat less developed, lie closer to 

 the body, and evidently do not function as swimming-organs — 

 compare the plicated edge of tlie pleuropodia of fig. 2, pi., 

 with that of fig. l,pl. Figs. 3 and 4, 2)1/ , indicate the line 

 of attachment of the pleuropodia to the body. The difference 

 between the pleuropodia in the two species is most marked at 

 their anterior end. (3) The colouring differs somewhat : in 

 A. 2}'iperata there is a uniform sprinkling of black dots all 

 over the animal except on the sole of the foot and under the 

 mantle, showing an inclination, especially on the head and 

 mantle, to run into small radiating lines. (The lighter 

 posterior end and dark encircling band described by Mr. Smith 

 are perhaps due to accidental causes, as another and better 

 preserved specimen in the collection shows no traces of 

 these.) In A. Mouhoti this speckling of dark spots is absent, 

 and there is a tendency rather to reticulate marking on pleuro- 

 podia and linear marking on head and mantle. 



The two animals, on the other hand, possess several striking 

 features in common. This is most marked in the general 

 external topography of the body, a point which I have else- 

 where tried to show is of special significance in the classifica- 

 tion of the Tectibranchs : — (1) The pleuropodia in both cases 

 start from about the posterior end of the first third of the body 

 and run backwards to within a few millimetres of the end of 

 the foot, being quite separate throughout their entire length. 



(2) In both the mantle, shell, and visceral mass are much 

 more posterior than in, e. g., A. limacina; and, in coordina- 

 tion with this, the genital opening is peculiar in being located 

 somewhat anteriorly to the mantle-cavity (figs. 3 and 4, ^.o.). 

 In the Tectibranchs it is, as a rule, within the pallial cavity. 



(3) The most striking point of agreement, however, is found 

 in the position of the rhinophora. These are situated close 

 together, just between the anterior ends of the pleuropodia. 

 This is such a marked feature, and is so different from what 

 is found in other Aplysiidaj, that it would seem to justify the 

 establishment of a separate genus. 



