THIil MOLLUSCS OF THE GEBAT AFRICAN LAKES. 163 



have traced a similar groove, faintly representing the spermatic 

 structure relating to the penis iu the male (PI. 15, fig. 15, g. v.). 



In none of these cases, however, do these grooves open into 

 an internal chamber; and since in the female they can serve 

 no conceivable purpose, unless it be that of oviposition, we 

 must conclude that the groove, where it exists in those Proso- 

 branchs which do not possess a pouch, represents a vanishing 

 structure, and that the pouch, although it was probably pos- 

 sessed by the ancestors of these forms, has been completely 

 lost. In some forms, moreover, such as Ty phobia, all trace 

 of this complex accessory reproductive apparatus has entirely 

 disappeared, and there is no trace of either groove or pouch in 

 the female or the male. 



In Tanganyikia, as in those forms of Melania which 

 possess the complete female apparatus I have described, there 

 is no penis, and only a slight spermatic groove represents the 

 apparatus in the male. But in Stronibus (PI. 16, fig. 3, g.v.) 

 and Pteroceras the penis exists, and the groove is not only 

 pronounced in these genera, but, as is well known, it is pro- 

 longed along the posterior surface of the organ to the tip. 

 Modifications of the male intromittent apparatus can, however, 

 go even further than this. In some forms, such as Buccinum 

 (PI. 16, fig. 4), there is no spermatic groove, nor indeed does 

 the vas deferens open in the mantle cavity at all. In these 

 forms the grooves have become enclosed, and as a simple 

 tubular continuation of the vas deferens pass through the 

 whole internal length of the penis, and open at the tip. (Com- 

 pare figs., diagram I, PI. 17.) 



Various modes of modifi.cation of the parts of the complex 

 apparatus existing in Tanganyikia andMelania episcopalis 

 are thus seen to be widely distributed among entirely different 

 prosobranchiate forms ; and although, so far as is at present 

 known, the entire apparatus exists fully developed only in the 

 four species I have named, it must be concluded, from the fact 

 that traces of these structures exist in most widely separated 

 types, that they are representatives of an archaic condition 

 of the reproductive apparatus, and we may almost certainly 



