1904.] PROM EAST AFRICA AXD ZANZIBAR. 271 



concei'ned. The shape and colour of the living animals are both 

 very variable. The former can be altered at will from an almost 

 circular to a long slug-like shape, in assuming which latter the 

 mantle-edges are folded over the back. The colour varies even 

 in the same animal, and individuals kept in captivity become 

 conspicuously paler and dullei' in a few hours. As is usually the 

 case with soft Nudibranchs, the preserved specimens are much 

 subject to distortion in alcohol, and such characters as the 

 expanded or contracted state of the branchia? and the flat or arched 

 shape of the back appear to have no specific importance. I think 

 that H. digitatus described by me (I. c.) is a distinct form, but its 

 state of preservation is such that it is difficult to say whether it 

 should be referred to Hexahranchus or to a new allied genus. 

 None of the other specimens which I have examined show any 

 material differences in structure or anatomy. The active 

 habits of the animals perhaps explain how it is that one varying 

 form is spread over so large an area. 



The numerous specimens which I have collected in Zanzibar 

 and on the East Coast of Africa represent three varieties : — 



(1) H. fcmstus B. — This form, which is not very abundant, 

 seems characterised by its prevailing red coloration and the 

 absence of any white bands. One specimen was of a dark blood-red 

 all over, with only a few yellowish markings at the sides of the 

 visceral mass, but in most cases this dark blood-red is confined to 

 a fairly broad irregular border. The centre of the back is of a 

 lighter red, with mottlings of various tints of red and orange. 

 Between this region and the border the colour is of a. dull i-eddish 

 grey. The specimens obtained of this variety are of moderate 

 size, not exceeding 8 centimetres in length. 



(2) H. marghiatus (Quoy &, Gaimard). — This variety, which 

 is also not veiy common or veiy large, is characterised by having 

 a. broad red band round the mantle, divided in the middle b}'- 

 a white line. In addition to this there is sometimes, but not 

 always, a white edge to the mantle. The middle of the back is 

 mottled red and orange. 



(3) The third variety, which is also the commonest, is the 

 animal captured by Moebius at Mauritius, and described by Bei'gh 

 (S. R. xvi. p. 828 ff.) as H. marginaUis (Q. & G.). Though it is 

 probably not specifically separate from H. marginatus, Bergh 

 seems to have overlooked the fact that it constitutes a variety 

 quite as distinct as the other so-called species. Its chief charac- 

 teristic is clearly given by Moebius : — " Die obere Seite des 

 Mantelgebrames nacli innen porzellan weiss, nach aussen niit 

 breitem rothem Saume von Avelchem abwechselnd kleinere und 

 grbssei'e Bogen nach innen iaufen." Though the diflference 

 between this form and H. faustus is really only one of degree, 

 the brilliant white band between the equally brilliant deej) red 

 border and the variegated central region is most conspicuous, and 

 the animal looks superficially quite distinct from those described 

 above. I propose to call it var. moebii. 



This variety is very common and grows to a very large size, 



