238 
THE TANGANYIKA PROBLEM. 
majority of the halolimnic forms. Bythoceras is at present 
represented in Tanganyika by two species, B. iridescens 
(Fig. 21) and B. minor (Fig. 24), both of which I 
dredged living at great depths* in the southern portion 
of the lake. Taking Bythoceras iridescens as our type, 
we find that when young the shell does not possess the 
characteristic spines. Nor has it the peculiar pearly 
thickening of the mouth invariably present in the older 
Fig. 21 . — Bythoceras iridescens. The shell front and hack (X J). 
forms. In the young condition the shell is extremely 
similar to that of Paramelania , and I am inclined to 
think that the figure of Paramelania crassilabris , given 
by Professor E. von Martens in his work, “ Beschalte 
Weichthiere, Deutsch Ost-Afrikas ” (PI. VI. Fig. 38), is, 
in reality, that of a young Bythoceras iridescens. 
The outward appearance of the animal is extremely 
similar to that of Cerithium viilgatum , with the exception 
* From 300 to 1,000 feet. 
