238 UNIONIDiE. 



few, though specimens are abundant ; in North America both 

 species and individuals abound. 



Like other fresh-water shells, the Naiades are often extensively- 

 eroded by the carbonic acid dissolved in the water they inhabit. 

 This condition of the umbones is conspicuous in the great fossil 

 TJniones of the "Weald en, but cannot be detected in the Cardinige, 

 and some other fossils formerly referred to this family. 



The outer gills of the female Unio are filled with spawn in the 

 winter and earlj^ spring; the fry spins a delicate, raveled byssus, 

 and flaps its triangular valves with the posterior shell-muscle, 

 which is largely developed, whilst the other is yet inconspicuous. 

 The shells of the female river-mussels are rather shorter and 

 more ventricose than those of the males. 



Over 1200 recent species of Unionidge are known to science 

 and more than half of these are inhabitants of the rivers of the 

 United States. A large proportion of the species was first 

 described bj^ Dr. Isaac Lea of Philadelphia, who has devoted 

 over fifty years to the study of this family. His " Observations 

 on the genus Unio, with descriptions of new species," etc., now 

 comprises thirteen quarto volumes, illustrated by hundreds of 

 beautiful plates. 



Unio, Retz. 



Etym. — Unio, a pearl (Pliny). River-mussel. 



Syn. — Uniomeris, Conr. (Eocene). 



Distr. — 1000 sp., universal. Fossil. Cretaceous, Eocene — ; 

 Europe. Triassic — ; N. Amer. C7. Z2YtoraZis, Linn, (cxxiii, 86). 



Shell oval or elongated, smooth, corrugated, or spiny, becom- 

 ing very solid with age ; anterior teeth 1-2, or 2-2, short, irregular ; 

 posterior teeth 1*2, elongated, laminar. 



Animal with the mantle-mai*gins only united between the si- 

 phonal openings ; palpi long, pointed, laterally attached. 



The subgenera are mainl}'^ founded upon peculiarities of form 

 and ornamentation or sculpture ; the}^ are of no value except as 

 conveniences for classifying the species ; and Dr. Isaac Lea, the 

 great authority upon this family, has discarded them altogether 

 and used instead, in his admirable " Synopsis of Unionidse," a 

 division into sections by the form of the shell, and these into 

 subsections by the sculpture. 



I annex the so-called subgenera, as adopted by H. and A. 

 Adams, Chenu and others, 



BARiosTA, Rafinesque, 1831. (Potamida, Swainson, 1840.) 

 Shell arcuated, smooth. U. emarginatus, Lea (cxxiii, 15). 



NAiDEA, Swainson, 1840. Shell obovate or mytiliform, smooth. 

 U. Modioliformis, Lea (cxxiii, 76). 



NAiA, Swainson, 1840. Shell oblong, smooth. U. depressus, 

 Lam. (cxxiii, 17). 



