246 JRcmarJiS on the JJnio Snyii and Uiiio Camptodon. 



thin, beaks sliglitlj prominent, incurved, and divergingly wrinkled; 

 cardinal teeth oblique, dngle in the right and double in the left valve ; 

 lateral teeth slightly curved ; nacre white. Diameter, 1; length, 1.6; 

 }>readth, 2.8 inches. 



Shell inequilateral, transverse, subrliomboidal, compressed ; poster- 

 ior and superior margins rectilinear ; basal margin curved ; anterior 

 margin regularly rounded ; valves thin, translucent ; beaks slightly 

 prominent, incurved, and divergingly wrinkled, placed near the anter- 

 ior margin ; umbonal slope subcarinate, carina somewhat elevated ; 

 ligament long, nari-ow, nearly straight and partially concealed ; epi- 

 dermis pale yellow, inclining to cupreous on the umbos ; glabrous, 

 with indistinct capillary rays of a lighter color extending over the 

 whole disk ; lines of growth black, and very distinct ; two faintly im- 

 pressed lines diverging from under the points of the beaks and extend- 

 ing to the posterior basal margin; cardinal teeth very oblique", not 

 prominent, single in the right and double in the left valve, slightly 

 crenate ; lateral teeth lamellar, slightly curved ; anterior cicatrices 

 distinct, posterior confluent, dorsal situated horizontally across the cav- 

 ity of the beaks and distinct ; cavity of the beaks shallow and rounded ; 

 nacrie white, slightly iridescent over the entire surface of the valve, 

 with faintly impressed strife or rays diverging from the cavity of the 

 beaks and extending to the basal margin. 



Since Mr. Say mistook the anterior for the posterior margin, and 

 the posterior for the anterior margin of our fresh water bivalves, this 

 must be taken into consideration when reading and comparing his 

 description of a shell with that of other writers. 



After the description of Unio campfodon, Mr. Say makes the fol- 

 lowing observations : 



"This interesting new shell was sent to me by Mr. Barabino, who 

 discovered it ojiposite to New Orleans, in ponds. An exterior view 

 of the shell would not immediately distinguish it from Alasmodonta 

 edentula, nobis, but the inner surface is quite different, and the arma- 

 ture of teeth separates them genericall3\" 



Upon a careful comparison of the descriptions here given, and an 

 equally careful inspection of the shells, we find numerous points of 

 difference between them ; differences so great as to enable us readily 

 to distinguish either shell, and to select them easily from all other 

 shells known. Our attention is necessarily directed, in the first place, 

 to the general contour of each shell and its external appearance ; then " 

 to the interior surface of the valves. In the Unio camptodon we ob- 

 serve a slight depression on the disk, which is continued to the basal 

 margin, causing tl\e shell to be slightly arcuate, its length really 



