Occasional Papers of the Museum of Zoology 3 



states, extending from Texas to British America, however, 

 are to be found in very considerable abundance what are evi- 

 dently the prototypes of our recent species. These fossils 

 show great variation, some of them are exceedingly similar 

 to recent forms, and others are diverse types which have ap- 

 parently become entirely extinct. Of course, we only know 

 these species from their shells. There is no evidence what- 

 ever as to the peculiarities, if any, of the animal that lived 

 inside of the shell. But comparing these fossil species with 

 the recent fauna we find that, with one exception hereafter 

 to be more fully considered, there has been, so far as the 

 shell is concerned, no change of any great extent. 



There being no evidence as to what were the peculiarities 

 of the animals of these fossil species, we are forced to judge 

 as to what was the course of evolution in the animal from the 

 various forms of development which we find represented in the 

 recent species. Taking the modern Unionidae as a whole, we 

 find that their organization is comparatively simple and. witli 

 one exception, very similar throughout the whole world. The 

 one feature in their anatomy which is a variable one is in 

 the adaptation of the gills as marsupia for carrying the eggs 

 until they are hatched. Assuming that the most primitive 

 form represented in the recent fauna is that of the genus 

 Margaritana, and this assumption is based not only upon the 

 peculiarity of its structure, but upon its range at the present 

 time, which is greater than any other genus and, with the 

 exception of the central part of British North America, ex- 

 tends entirely around the world, we find that all four gills 

 are used as marsupia. The eggs, passing from the ovaries, 

 occupy all four gills of the animal. The gills have neither 

 water-tubes nor septa. The interlaminar connections are 

 patch-like, irregular in shape and position, with indications of n 



