THE MOLLUSCS OP THE GREAT AFRICAN LAKES. 197 



sessed by the forerunners of the Meianias, the Paludinas, the 

 Bythinias, or indeed any of the recognised fresh-water types. 

 Therefore^ so far as the nerves go, the anatomy of the Typho- 

 bias gives a flat contradiction to the view that these Gastropods 

 may be the survivors of any extinct fresh-water stock .^ The 

 nervous system of Ty phobia exhibits, on the other hand, the 

 characters of some ancient but more especially of several 

 modern marine genera. Therefore the evidence which can be 

 gathered from the anatomy of the nerves is exactly in accord 

 with the deductions which were drawn from the study of the 

 distribution of these Gastropods, the Typhobias appearing to 

 be among the survivors of some old, but not geologically 

 ancient, marine types.^ What is true of the nervous system is, 

 however, true of the remainder of the soft parts. Beginning 

 with the digestive organs, it will be seen on reference to 

 fig. 43 that the Typhobia radula, although very singular and 

 self-contained, is still comparable to that of several marine 

 Tsenioglossa. Thus in the massive characters of the admedian 

 teeth, the long slender character of the laterals, as well as the 

 form of the median tooth, this radula approaches to those 

 of Chenopus, Zenophora, Trochiformis, Pteroceras, 

 Strombus, and Pustularia, while it has many characters in 

 common with the radulce of Crepidula, Trochita, Hy ponix, 

 Turritella, and Cassis, and it resembles in a less degree 

 those of Vermetus, Triton, Ranella, and Natica. 



The characters of the salivary glands, the relation of the 

 stomach to the oesophagus, of the intestine to the stomach, the 

 position of the apertures in the stomach as well as the cha- 

 racter of the pronounced median fold in the posterior stomachic 

 chamber, are all characters which are strictly analogous to 

 those obtaining in the Strombi and Pteroceras. The crystal- 



^ The view that some of the Halolimnic forms are the remains of an old 

 fresh-water stock, as advocated by White, Tausch, aud others, will be found 

 discussed in my paper on distribution, loc. cit. 



" I beg that I may not be misunderstood in this : it is one thing to say that 

 the Typhobias are old, since the lake in which they now live must have been 

 cut off from the sea for a great many years ; it is quite another thing to say 

 that the Typhobias were contemporary with geologically ancient forms. 



