Notes on South African Mollusca. 123 



slight median projection. The radula would have nearly straight 

 rows of "quadrate" teeth of the ordinary ITelicid type, and the 

 alimentary canal would pursue the usual course, though the differ- 

 ence in size between the two divisions of the liver would be less 

 marked than is the case in most snails at the present day. The 

 reproductive organs would be of a primitive type. A well-developed 

 vesicula seminalis would be present, and there would be a swelling 

 near the junction of the free oviduct and the duct of the sperma- 

 theca a swelling which may have grown out into an appen- 

 diculum ; but the spermathecal duct would have no diverticulum, 

 and there would be no other accessory outgrowths of any kind. 

 Though the penis and vagina would open together, a common 

 vestibule would not have been developed. A considerable portion of 

 the vas deferens, instead of lying freely in the body-cavity, would be 

 still partially embedded in the invaginated portion of the skin which 

 forms the wall of the penis. The retractor of the right upper 

 tentacle would pass between the penis and the vagina. 



Such we may suppose to have been the main characters of the 

 earliest members of the Acavidae. It will be seen that they were 

 more primitive than many modern snails, but probably at the 

 beginning of the Mesozoic era they were both larger and more 

 highly organized than most of the other families then living. And 

 the family increased and multiplied, and spread far and wide through 

 that ancient southern continent. Moreover, both their multiplication 

 and their spreading led to the further evolution of these snails. 



As the individuals increased in number under the favourable 

 conditions, they would become more crowded, and the competition 

 between them would become more severe. This would lead to the 

 evolution of higher types : unfavourable varieties would be elimi- 

 nated ; progressive mutations would alone survive. Thus the eggs 

 grew larger and larger, or, in an extreme case, the animal became 

 viviparous, the embryo reaching a relatively enormous size. The 

 adult also became larger and broader, the whorls very much wider, 

 and the apex more and more obtuse, until the form of the shell 

 became globular, and finally depressed. The lip also became more 

 expanded. A closed primary ureter was developed, and eventually 

 the beginning of a secondary ureter.* The fine striae on the jaw 

 either developed into ribs, or, more frequently, disappeared altogether. 

 Similarly, the comparatively useless small secondary cusps on the 

 teeth of the radula were generally eliminated, the main cusps 

 becoming larger and broader at their expense. The reproductive 

 * See Randies, Proc. Mai. Soc., 1900, iv. PI. IX, fig. 1. 



