﻿INTRODUCTION V 



are contained in all four of the branchicr, and this would place 

 this genus in the group I have called "Tetragcnecc." The char- 

 acters of the shells of Parreysia would indicate that they 

 should be placed in Rosanoramphus as they have mostly zig- 

 zag radial beak scalpture. It is quite probable that this and 

 some other genera miay be connecting links between the two 

 great groups. 



Gravid material of two South African species has been 

 examined by tlie author since writing the Synopsis. These 

 and allied species had been placed provisionally in Nodularia, 

 but they seem to be true Unios. None of the South African 

 shells of this group that I had examined were in condition to 

 show the beak sculpture. 



The attempt to systematically classify much of the Asiatic 

 and South African material and some of that of South America 

 was largely tentative and it is quite likely that many changes 

 will have to be made when more and better conchological and 

 anatomical material can be carefully studied. When we are 

 able to examine gravid material of all the larger groups, then 

 and not till then will it be possible to be certain of the relation- 

 ship of a large number of forms. 



The only guide to classification in a great many cases w'as 

 the shell, and such material as was accessible was often in 

 bad condition or totally insufficient in quantity and the beaks 

 in nineteen cases out of twenty were so eroded that it was 

 impossible to determine anything accurately from them. In 

 other cases only a figure and a description were accessible and 

 in still others only a description. The character of dimor- 

 phism is a very important one, but to be of much use to the 

 student, he should possess male and female shells of species, 

 which are so- developed. For example, in the genus Ncphro- 

 noias many of the male shells cannot be distinguished from 

 the shells of Unios by any means I know of, but the female 

 shells of such species differ from the males in possessing a 

 marsupial swelling, while those of Unio do not have it. 



The following table taken from the Synopsis will show the 

 classification of the genera and higher groups. 



