238 Annals of the Carnegie Museum. 



there is or has been a general tendency to restrict the marsupium to 

 the i mter gills, and that this forward step in a quite natural direction 

 has been made independently in various groups. That is to say, the 

 Unio-type of marsupium has repeatedly developed from the Quadrida- 

 type by parallel evolution. 



Of the other features of the soft parts only three furnish some help 

 for the distinction of genera. The first and most important is the 

 character of the placenta?, revealing differences which are of prime 

 value, but affect only a few forms, as will be seen below. The second 

 is the separation of the anal and supra-anal openings. Although 

 characteristic of certain forms (in one case these openings are not at 

 all separated), it is somewhat variable in others, even individually. 

 Thus we can use this character only to a limited degree. The third 

 is the connection of the inner lamina of the inner gills with the ab- 

 dominal sac. Here there seems to be a difference between certain 

 forms of the Old and the New World. But, unfortunately, too few of 

 the former are known for me to express a final judgment. 



Thus the soft parts alone would furnish only few criteria for the 

 distinction of genera, and we should direct our attention to the shell. 

 Here we have indeed great variety, and the shapes of the shell have 

 been largely used heretofore for the definition of genera. The most 

 important feature, in my opinion, is the beak-sculpture, which, how- 

 ever, has been largely misunderstood by Simpson. In fact in this 

 primitive subfamily we have, side by side, all the different types 

 of beak-sculpture, and, as we shall see, they may be used to great 

 advantage. 



Since various types of shell-structure are frequently combined with 

 various types of soft parts, it would not do to make only a lew large 

 generic divisions. For if we recognize, for instance, only two main 

 genera according to the character of the marsupium, the same types 

 of shell would turn up in either of them, which surely would give an 

 incomplete or wrong impression of affinities. Thus, in my, opinion, 

 it. is advisable to admit a larger number of genera founded upon both 

 the structure of the soft parts as well as of the shells. Such a scheme 

 is introduced here, at first, tentatively, but I hope it finally will prove 

 to be the most convenient. 



Finally I should mention the glochidia of these forms. The latter 

 are known in a number of North American species, where they always 

 are of a primitive shape (see Plate XIX, fig. i). They are also 



