FAMILY UNIONISE ANODON. 199 



GENUS ANODON. Bruguieres. 



Animal as in the two preceding genera. Shell generally thin ; hinge toothless ; all the other 

 characters of the two preceding genera. 



Anodon unadilla. 



PLATE XV. FIG. 228. 



(CABINET OF DR. BUDD.) 



Description of the adult shell, solid, concentrically rugose, (more particularly on the posterior 

 portion), transversely subelliptical, kidney-shaped, inflated, inequilateral. Beaks large, elevated, 

 contiguous, very prominent, anterior to the centre of the shell : greatest diameter near the centre 

 of the shell. Hinge-margin slightly arched, nearly straight : upper posterior margin sloping 

 to the regularly rounded posterior margin ; basal margin widely arcuated and compressed 

 on the side ; anterior margin broadly and regularly rounded. Within, the cavity is capacious ; 

 in the beaks, deep and wide, with a crescent-shaped deep cicatrix far within : palleal impres- 

 sion very distinct. Anterior cicatrices confluent ; posterior distinct, the upper very small, and 

 placed immediately under the end of the hinge-ligament ; dorsal cicatrices five, very conspi- 

 cuous, small, and arranged in a regular series anterior to the cavity of the beak. 



Color. Epidermis dark brown, passing into dark olive green on the basal margin ; beaks 

 yellowish brown : within, salmon-color, brightest within the limits of the palleal impression ; 

 margin bluish white. 



Vertical axis, 2-0; transverse ditto, 3*5. Diameter, 1"5. 



This species is an exception to the old generic character, as it is remarkably stout and solid. 

 It was obtained by Dr. C. H. Stillman, from Unadilla river, Otsego county, a tributary of the 

 Susquehannah. In its general outline it resembles A. cylindracea of Lea, but is at once 

 distinguished by its solidity and greater inflation, and the situation and prominence of its 

 beaks ; the palleal impression, in our specimens, may be traced through the posterior cica- 

 trices. In the smaller specimens, the beaks are distinctly undulated ; the epidermis is darker, 

 and the nacre is of a deeper salmon-color : the palleal impression in all may be traced through 

 the posterior muscular impressions. 



