REPRODUCTION AND ARTIFICIAL PROPAGATION OF FRESH-WATER MUSSELS. II 7 



can species belong to the former group, while the latter contains forms that are found 

 chiefly in Asia, Australia, Africa, Central America, and South America." 



As our observations have been confined to the Exobranchiae, reference will be 

 made only to this group, the following subdivisions of which are recognized by Simp- 

 son, each distinguished by special marsupial characters: 



Tetragense: Marsupium occupying all four gills. 



Homogenae: Marsupium occupying entire outer gills. 



Diagense: Marsupium occupying entire outer gills, but differing from that of the 

 Homogenae in that the egg masses lie transversely in the gills. 



Hcterogenae : Marsupium occupying only posterior end of outer gills. 



Mesogenae: Marsupium occupying a specialized portion in the middle region of 

 outer gills. 



Ptychogenae: Marsupium occupying entire lower border of outer gills which is 

 thrown into a series of peculiar folds. 



Eschatigenae : Marsupium occupying the lower border only of outer gills, but not 

 folded. 



Simpson has established another group, the Digenae, for the genus Trilogonia, but 

 since its marsupium is constituted by all four gills (Sterki 1907), it should at least be 

 included in the Tetragenae, if not in the genus Quadrula, as Ortmann maintains (1909, 

 191 1). For a complete list of the genera occurring in each of Simpson's groups, refer- 

 ence may be had to his Synopsis (op. cit., p. 5i4-5i5)- 



These groups constitute Simpson's subfamily, Unioninae, his other subfamily, 

 Hyrianae (Hyriinae), coinciding with the Endobranchise or those Unionidte whose mar- 

 supium occupies the inner gills only. In all of the Unioninae except the Heterogense 

 and Digenae (Tritogonia) , according to Simpson, the sexes are indistinguishable exter- 

 nally. 



It will be seen from the above classification that three general conditions exist 

 in the Unioninae, namely, one in which the marsupial adaptation involves all four 

 gills; one in which the entire outer gills only are utiUzed; and, lastly, one in which 

 some differentiated portion of the outer gills constitutes the marsupial region. It 

 would, accordingly, be a more logical procedure to make these general marsupial con- 

 ditions the basis of the classification and to recognize only three main groups corres- 

 ponding to the three general types of marsupium, to which the names Tetragense, 

 Homogenae, and Heterogenae might be applied; and since all of the remaining forms 

 have a marsupium which may be readily regarded as a secondary modification of one 

 or another of the three types, they could be arranged in appropriate subgroups. If 

 this were done, the Diagenae would obviously fall within the Homogenae, while the 

 Mesogenffi, Ptychogenae, and Eschatigenae would be placed under the Heterogena, as 

 in all of the latter forms the marsupium is some specialized portion of the outer gills. 



<■ Besides the Unionidae, a second family, the Mutelidae. is recognized by Simpson in his classification of the Naiades or pearly 

 fresh-water mussels. In these forms, which belong to Africa and South America, the marsupium is the irmer gills only, and the 

 larva is not a glochidium but the so-called lasidium. The genera embraced in this family are not considered in the present account. 



