The Tanganyika problem; an account of the researches undertaken concerning the existence of marine animals in Central Africa . r where the eggs go throughtheir later stages of development, the animal being vivi-parous. SPEKIA, CROSS. S. ZONATA. (FIG. 39). The shell of this remarkable mollusc is representedin Fig. 39, and it is certainly most curious that noattention has been drawn by any of the conchologists to THE TANGANYIKA PROBLEM. 257 the extremely naticoid character which it presents, for theshell of this species is so completely similar to that ofnumerous fossil naticoid forms that, had
The Tanganyika problem; an account of the researches undertaken concerning the existence of marine animals in Central Africa . r where the eggs go throughtheir later stages of development, the animal being vivi-parous. SPEKIA, CROSS. S. ZONATA. (FIG. 39). The shell of this remarkable mollusc is representedin Fig. 39, and it is certainly most curious that noattention has been drawn by any of the conchologists to THE TANGANYIKA PROBLEM. 257 the extremely naticoid character which it presents, for theshell of this species is so completely similar to that ofnumerous fossil naticoid forms that, had it appeared fos-silised instead of having been found living in a great fresh-water lake, there is not the slightest doubt that it wouldhave been placed in one of the numerous fossil generawhich are supposed to group themselves about the livingNaticas. The oblique aperture, and the tendency of theouter wall of the mouth to be continued as a cup-shapedring round the bases of the older shells, are exactly what isobserved in many fossil Naticas ; while the presence of avery pronounced umbilical opening, which is more or less. Fig. 40.—Lingual dentition of Spekia zonata, filled up with a deposit of callous substance, are featureswhich are generally regarded as almost diagnostic of naticoidshells. The external appearance of this form is superficiallysimilar to that of Tanganyicia rufofilosa. The foot israther less broad, and the snout is not so much pigmented ;but, apart from the naticoid appearance of the shell, it isonly in the internal anatomy that we begin to appreciatethe wide morphological differences which exist betweenthese forms. In Spekia zonata the buccal mass is well developed, andthe radular sac is conspicuous, but not of any considerablelength. There are two very strong muscles attaching thebuccal mass to the body wall, and the salivary glands are 17 258 THE TANGANYIKA PROBLEM. long and simple. The radular dentition is characteristic,and very strongly developed. A sin
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