GEOIvOGY OF LA SALIvE COUNTY. 



117 



We follow Lea in his division according* to form 

 and surface of shell. 



1. Winged, the upper edg-e of shell extended 

 back into a kind of wing*, triangfular, smooth. 



U. alatus. Three and a half to four inches long*, 

 width the same, brown without, thin, flat, 

 white or purple within, muddy bottom. 



U. g-racilis. Four to five inches long*, width same, 

 lig-ht brown, very thin, flatfish, bluish white 

 within. 



2. Rectang'ular, long*er than wide, surface ribbed 

 or plicate, posterior marg*in often wrinkled, dark. 



U. plicatus. Five and a half Iv., four W., shell 

 thick and heavy, with about five strong* wrin- 

 kles running* from the umbones to the poste- 

 rior marg-in, dark brown or black. 



U. multiplicatus. Four and a half L., four and 

 a half W., leng*th and width about the same, 

 posterior marg*in much wrinkled, black or dark. 



U. undulatus. Oblong*, with four or five strong- 

 undulations running* diag-onally across the shell, 

 leng*th twice the breadth. 



3. Warty or pustulate. 



U. pustulatus. Rectang*ular, two and a half L., 



two and a half W., thick, yellowish brown, 



covered with warts or nodules. 

 U. dorfeuillianus. Triang-ular, in color and size 



like Pustulatus. 

 U. iris. — Long*, flat, 2}4L., obliquely rayed with 



g*reenish bands. Not common. 

 U. g-ibbosus. — Heavy, thick, dark brown, bluish 



within. 



U. capax. Oval, brownish g*reen, 3}4L/., 2%W., 



thick, inflated, rayed with g*reen. 

 U. cariosus. Oval, dull yellowish g*reen, 4L., 



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