July i, 1897] 



NATURE 



199 



became at once turned towards the Tanganyika fauna as a 

 whole, and a re-examination of the shells of the molluscs 

 of the lake, which had been brought home by various 

 travellers, showed that the medusa was only one member 

 of a remarkable fauna, many of the forms of which were as 

 singular and marine-looking as the jelly-fish itself Up to 

 the present time, however, the real nature of this remark- 

 able assemblage of lake animals has of necessity remained 

 entirely problematical, since, without more material, it 

 was impossible to attempt to determine whether the close 

 similarity, which some of the gastropod sliells exhibit to 

 species living elsewhere in the ocean, was due to actual 

 affinity, or was brought about by a convergence of 

 variations induced somehow in an originally fresh-water 

 stock. To obtain the material sufficient to throw light 

 on these forms, by a determination of their actual 

 affinities, was the object of an expedition to Tanganyika, 

 which, through the generosity of the Royal Society 

 and British Association, I was able to undertake nearly 

 two years ago. 



The only practical route at present open to the Lake 

 lies up the great Zambesi water-way, which, but for 

 the Murchison Cataracts, extends from the coast to the 

 north end of Lake Nyasa. From this point it is 

 necessary to march across the elevated forest land, and 

 mountain ranges which constitute the interior plateau. 



Fig. I — Living Typhobia Horei. 



Approached in this direction the lake becomes first 

 visible from the heights above Kituta, where a strip of 

 bright green marsh-land skirts its southern coast. De- 

 scending from these heights the damp oppressive heat 

 of Tanganyika is felt at once, and long before reaching 

 the water's edge the forest is filled with the sickly scent 

 which characterises the whole lake, and reminds one 

 strongly of that of a weedy tidal beach. The lake has 

 fallen considerably of late, and much of the coast-line is 

 now obliterated by dense papyrus swamps, reeds, and 

 partially submerged mimosa scrub, so that its actual 

 shore is often very difficult to approach. In the deep 

 water of this southern arm there are vast drift collections 

 of empty Neothauma shells, which act as growing points 

 for sponges, and constitute the feeding-ground of in- 

 numerable small active and entirely aquatic crabs. The 

 living Neothaumas are found in shoaler water, generally 

 on the flat sandy deposits which characterise the coast- 

 line of other portions of the lake. Above the water's 

 edge this sand has been thrown up by the wind and surf 

 into low dunes, which are now covered with mimosa 

 scrub and wild cotton, and beyond these, again, there is 

 generally a strip of swamp swarming with Dactylethera, 

 Ampularia shells, and frogs. 



In many places, however, the mountains are not 

 fringed by these lacustrine flats, and where the great 

 NO, 1444. VOL. 56] 



western escarpments of the rift valley in which Tan- 

 ganyika lies, rise perpendicularly from the water's edge, 

 the submerged stones are covered with a bright growth 

 of green alga", and studded with numbers of the so- 

 called Paramelania, a marine-looking gastropod, the 

 affinities of which are not yet known. The great range 

 of variation vyhich these shells exhibit is most remark- 

 able, and their differentation seems to be a simple func- 

 tion of the depth at which the shells exist, those with 

 the most prominent processes being the lowest. 



The different kinds of coast-line, which I have just 

 described, are more or less characteristic of the whole 

 lake, and each has a fauna peculiar to itself; but, on the 

 flat sandy beaches, all sorts of shells, fish-bone/s, and the 

 like are thrown up together by the waves, so that it is 

 some time before one ascertains the habitat of each. 

 Thus I observed the exquisite spined Typhobia shells 

 two months before I found a specimen alive, owing to 

 the fact that Typhobia, together with some curious 

 associated forms, inhabit the profound depths of the lake, 

 and are only to be obtained by dredging with lines of 

 frorn five to seven hundred feet. 



Like Neothauma and Paramelania, these deep-water 

 gastropods are all viviparous ; but while the first deposits 



Fig. 2. — Sponge growing on dead shell of Neothauma. 



only two or three large embryos at once, Typhobia 

 produces a great number of bright green young. In this 

 form all stages of development are present in the same 

 animal, and we have, therefore, enibryological material 

 which may throw much light on the affinities of an 

 aberrant group. It would be unprofitable for me, as yet, 

 to enter into any discussion of the comparative mor- 

 phology of these forms ; but it may be interesting to 

 note that, while the radula formula of Neothauma 

 approximates in character to that of Paludina, that of 

 Typhobia and its associates is almost unique, and 

 although the Paramelanias show some similarity, in this 

 respect, to the forms with which they have been pro- 

 visionally associated, the radula of the so-called Lytho- 

 glythus of Tanganyika is absolutely unlike those of the 

 forms to which it has been thought to belong. 



Typhobia and its allies inhabit the flat mud bottom 

 of the lake, and the mud itself contains numerous 

 spiculas, which, on treatment with nitric acid, are seen 

 to be siliceous, many of them being indistinguishable 

 from those of the sponge Potamolepis, found as an en- 

 crusting species in the Congo, but which I have not seen 

 living in Tanganyika. 



