FAMILY UNIONID^E — ALASMODON. 197 



elevated, compressed, carinate : posterior hinge-margin abruptly depressed, with numerous 

 obtuse oblique wrinkles near it ; the ridge from the beaks to the posterior margin distinct. 

 Teeth (one in each valve) compressed, slightly elevated, and terminating abruptly behind, 

 sometimes scarcely apparent. Surface with numerous concentric wrinkles behind. 



Color. Epidermis olive-green, with numerous darker green inteirupted radiations; within, 

 bluish white, with a tinge of buff in the centre. 



Vertical axis, 1*0; transverse ditto, 2*0. 



This species assumes great variety in shape and coloring, and is supposed by Mr. Lea to 

 be the same shell from the Western States, described by Mr. Say under the name of A. 

 truncata. It is found in various parts of this State. 



Alasmodon arcuata, 



plate xiv. fig. ss4. 



(STATE COLLECTION.) 



Alasmodonta arcuata. Barnes, Am. Journ. Sc. Vol. 6, p. 277, p!. 12, figs. 20 and 21. 



Margaritana margaritiftra. Lea, Trans. Am. Phil. Soc. Vol. 6, p. 13G. 



Alasmodon arcuata. Adams, Am. Journ. Sc. Vol. 40. Got/LD, Invertebrata, p. 114, fig. 75. 



Description. Shell thick and strong, subcylindrical, bent, or obscurely kidney-shaped. 

 Hinge-margin elevated, compressed, carinate. Anterior slope declivous, terminating in a 

 narrow somewhat pointed anterior margin. Beaks slightly elevated, very far on one side, 

 often much eroded : hinge-margin and basal margin usually parallel ; the latter (in old speci- 

 mens) much arcuated. Teeth in one valve double, erect, strong, one of them deeply grooved 

 so as to form a slight denticulation on its edge ; in the other valve, the tooth is single, long, 

 grooved, and with a pit on each side : a slightly elevated fold in the place of lateral teeth. 



Color. Epidermis brownish black, loosely wrinkled towards the margins ; in young speci- 

 mens, smooth : within bluish white, iridescent ; margin greenish. 



Vertical axis, 2*0-2*6; transverse ditto, 4*0-5*5. 



This is one of the largest and most common of our Unios. Mr. Lea has thought proper 

 to consider it as identical with the Mya margaritifera of Europe ; but as Dr. Gould has 

 shown, that shell is shorter, the beaks more central and elevated, and the interior minutely 

 granulated. My specimens were from Rockland county, Champlain, Oneida, and many other 

 localities. 



