128 Annals of the South African Museum. 



continent of Asia. Indeed it is somewhat remarkable that the 

 family has survived even in Ceylon, for several of the more highly 

 developed forms have invaded that island. Probably the Acavinae 

 owe their survival to the protection and nourishment which they 

 give to their young until the latter have reached a large size. A 

 parallel may be drawn between the Acavinae and the Elasmobranch 

 fishes. The Elusmobranchs form a very ancient group, which is far 

 more primitive in most of its characters than the great majority of 

 modern fish ; yet they have survived to the present day and have 

 attained an unusually large size. This is probably due to the care 

 which they bestow upon their young : some have very big eggs 

 like Acavus, others are viviparous like Stylodonta, but, in both 

 cases, the young grow to a very large size before they make their 

 appearance in the world. 



We have now seen what happened to three of the four subfamilies 

 into which the Acavidae became divided, when their original home 

 broke up into four large islands. Only the African division remains 

 to be dealt with. 



The descendants of this branch of the family are now living in the 

 west of the Cape Province and in the neighbouring part of German 

 South-West Africa. As might have been expected, these snails are 

 much more primitive than the Acavinae, being further from the 

 centre of evolution. The eggs are not so large ; the kidney is without 

 an ureter ; the jaw is striated, and secondary cusps are always 

 present at least on the outer teeth of the radula. The reproductive 

 system is without an appendiculum, but the free oviduct is usually 

 much swollen, and the vas deferens is often closely bound to the 

 penis. 



Nor is it surprising to find that this subfamily has greater affini- 

 ties with the Strophocheilinae than with the Acavinae, for recent 

 researches have shown that the connection between Africa and 

 Brazil probably remained long after the formation of the Mozambique 

 Channel, although it is possible that Madagascar was reunited with 

 Africa for a short period during Tertiary times, after the trans- 

 atlantic connection had broken down. The resemblance between 

 Pilsbry's figure of the pallial organs of Gonyostomus multicolor, from 

 Southern Brazil, and the corresponding organs of Trigoneplirns is 

 undeniable ; :;c and the irregular longitudinal folds which he shows 

 inside the swollen free oviduct of the same South American species 

 occur also in the South African forms. Moreover, the radula of this 



* Cf. Man. Conch., 1902, xiv. PI. XLIX, f. 8, with PI. Ill, f. 1-4 in the present 

 work. 



