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THE TANGANYIKA PROBLEM. 
TAN (JAN VIC I A, CROSS. T. RUFOFILOSA (FIG. 28). 
Upon the shore rocks and flourishing among the surf 
and the breakers of Lake Tanganyika there are several 
species of small molluscs, which in the fauna of the 
lake fill the place of the Littorinas and Neritinas of 
the sea shores, and among these littoral forms there 
occurs in great abundance the animal, to the empty shell 
of which Cross gave the name of Tanganyicia. In its 
shell (Fig. 28) Tanganyicia somewhat resembles a small 
A r atica, but it lacks the callous characteristic of most, if 
Fig. 28. — The shell and operculum of Tanganyicia 
rufofilosa. 
not all, the shells of the true Naticas. In the general 
arrangement of the parts within the mantle cavity, the 
character of the snout, head, and tentacles, in the position 
of the renal, reproductive, and alimentary apertures, and in 
the character of its gill, this animal (Fig. 29) very much 
resembles Typhobia ; the osphradium is, however, somewhat 
foliated at its outer end, and in the female the rectum is 
provided with a large rectal gland. In both sexes the 
genital aperture is placed farther back than in any of the 
preceding forms, and in the male we encounter, for the first 
time among the halolimnic gastropods, an unmistakable 
