CHAPTER XXIII: THE PEARLY FRESH-WATER 



MUSSELS 



Family UNioNiDii 



Shell equi valve, pearly, with thick periostracum; hinge 

 variable, with or without teeth of schizodont type, all genera 

 show at least vestiges of teeth arranged as pseudo-cardinals and 

 laterals; beaks sculptured usually, often showing remains of 

 nuclear shell, eroded when old, as a rule; pallial line usually sim- 

 ple; prismatic border narrow; sculpture variable. Animal with 

 large foot; mantle borders free; gills four, leaf -like, dorsally at- 

 tached; labial palpi four, wider than long. Embryo a glochidium, 

 with bivalve shell, developed in the gills of the parent. Sexes 

 united or separate. In higher forms, shell of the female swollen 

 to accommodate the egg pouches, marsupia. Shells of thick 

 pearl used in button-making. Pearls of considerable value are 

 obtained from river clams. 



A vast and complex family of fresh-water clams, inhabiting 

 the sandy or muddy bottoms of streams and bodies of still water, 

 Every continent has its own genera; every great drainage 

 basin has its peculiar species. There is much variability within 

 species. 



Mr. Charles T. Simpson has published a "Synopsis of the 

 Naiades" in Volume XXII., No. 1205, of the Proceedings of the 

 United States National Museum, issued in 1900. Under the 

 name. Naiades he groups the families Mustelidae and Unionidje. 

 The latter only is represented in North America, though it is by 

 no means restricted to this continent. The Mustelidse include 

 eleven genera, in tropical Africa and South America. The 

 Unionidse comprise sixty-one genera, with about one thousand 

 species and eighty-two varieties. Of these over five hundred 

 species and fifty-five varieties belong to North America. Others 

 are distributed over the other continents. 



Besides significant differences in the hinge teeth the two 



373 



