1904.] FROM EAST AFRICA AND ZANZIBAR. 297 



spread out, and 4*5 high when they are closed and folded together. 

 The colour is yellowish green with black spots within and without, 

 the borders having disappeared. The shape and other external 

 characters are exactly as in the specimen just described. The 

 teeth are also similar but a triile more bent : there are 1 6 in the 

 radula, and about 15 of various sizes in the heap. 



I think these specimens must be referred to Elysia (Fterogastron) 

 marginata Pease, although his description is somewhat deficient 

 in details. In vieAv of the fact that the teeth of the animal 

 examined resemble those of E. nigrocincta, as described by Bergh, 

 and that the coloration is clearly very variable, it is probable that 

 both E. nigrocincta and viarginata are varieties of a protean 

 species ranging from green spotted with black, but without a 

 coloured border, to green with or without black spots, and a more 

 or less continuous single, double, or triple border. 



Elysia dubia, sp. n. (Plate XYII. figs. 14-17.) 



Four specimens from Chuaka, found on Zostera at low spring- 

 tides. The animals were dark green with a few spots of dull, 

 light blue. They were about 6 mm. long and 3 broad when at 

 rest, but when crawling become narrower and more elongated; 

 They can also swim on the surface of the water foot uppermost. 



As preserved, the specimens are of a uniform dark green, and 

 have somewhat the appearance of a minute Ajolysia, as the wings 

 do not reach to the end of the body but terminate separately, 

 leaving a distinct tail. The tentacles have become mere knobs, 

 but were apparently of a fair size in life. The foot is very 

 distinctly divided from the body by a ridge. The pericardium is 

 continued into a long median ridge down the centre of the body. 

 At about the point where the wings end it bifurcates. From each 

 side of this central ridge issue seven or eight vein-like ridges, 

 mtich as in Placohranchus. The radula consists of 14 teeth and 

 about 20 in the heap ; they are elongate and slender. The basal 

 part is nearly as long as the hook and there are no denticulations. 



The specimens are perhaps immature. 



If Elysiella is regarded as a separate genus, these specimens 

 should probably be referred to it, but Bergh (Beitr. zur !£enntniss 

 der Aeolidiaden, viii. 1886, p. 17) seems doubtful as to the 

 validity of the genus and its definition. For the present I think 

 it simjDler to refer this form to Elysia. 



EXPLANATION OF THE PLATES. 



PXATE XVI. 



Fig. 1 Phi/lUdia nobilis, dorsal view (p. 282). 



2. Hervia lineata, dorsal view (p. 286). 



3. „ „ one of the cerata. 



4. Vacelina lineata, dorsal view (p. 288). 



5. „ „ one of the cerata. 



6. Stiliger varians, three teeth (p. 290). The outline of the middle tooth 



is coloured red for distinctness. 



7. 'Elysia marginata, dorsal view with wings open (p. 296). 



8. ,, „ dorsal view with wings closed and bodj^ elongated 



