250 THE TANGANYIKA PROBLEM. 



same form as in TypJwbia and all the rest of the halolimnic 

 molluscs that we have considered, and as it is with the 

 alimentary canal, so it is with the rest of the animal's 

 visceral anatomy. 



NASSOPSIS, SMITH. N. NASSA (FIG. 33). 



During life this mollusc inhabits the surface rocks of 

 Tanganyika, and its shells are always richly encrusted with 

 the green algae which clothe the rocks for a consider- 

 able depth. It is sluggish, and appears to browse within a 



Fig- 33- — Shell of Nassopsis nassa. 



very limited area, like the Patellas of the ocean beach. 

 The foot is broad, somewhat pigmented, and quite white in 

 places ; the snout is broad, black, and wrinkled, not pro- 

 trusible, but retractile. The tentacles are short and black, 

 and the eyes are not carried on the tentacles themselves, but 

 on secondary papillae at their posterior bases. There is a 

 well-developed mucous gland in the mantle cavity, and the 

 animal, unlike the genus Spekia, is viviparous. There is a 

 tolerably well-developed buccal mass ; the radular sac is of 

 average length, and the salivary glands are somewhat 

 tortuous, simple saccular organs. The radular dentition 

 is strong (Fig. 36). From a portion of the radula 



