4 University of Michigan 



diagonal arrangement which does not follow the gill filaments 

 and does not form continuous septa. 



The next step apparently in the evolution of this feature 

 of the animal was the formation of water-tubes by septa run- 

 ning parallel with the gill filaments, all four gills, however, be- 

 ing still used for carrying the eggs. The next advance was a 

 very radical one, and resulted in the division of the Naiades 

 into two great families; in the one (the Mutelidse), which is 

 not represented in our northern fauna but is confined to the 

 southern half of the globe, only the inner gill is used for 

 carrying the eggs, while in the Unionidse proper, which in- 

 cludes the whole of the North American fauna, either all four 

 gills or only the outer gills are used for that purpose. All 

 the North American species of the recent fauna, which belong 

 to the subfamily Unioninse, which contains the more primitive 

 types, and in which either all four gills or only the entire outer 

 gills are used as marsupia, carry the eggs only a short time and 

 are what are called tachytictic genera. 



It would seem that, for some reason which we do not 

 now know, the next step in the evolution of these creatures 

 was for the purpose of enabling the female to carry the eggs 

 in the gills for a longer period, practically through what we 

 now call the winter season. These higher groups are called 

 brachytictic for that reason. The development for this pur- 

 pose was carried on in two different ways. In one great di- 

 vision, or subfamily, the Anodontinse, the necessity was met 

 by increasing the complexity of the water-tubes themselves. 

 Whereas in the more primitive forms the water-tubes were 

 filled with the eggs, and the Avater circulated through them, 

 in this particular subfamily there has been developed in the 

 water-tubes a series of longitudinal partitions which divide 

 each tube into three parts, an inner ovisac in which the eggs 



