CONCnOLOGICAL WRITINGS. TO 



* 3. I)ECURAMBts. Cardinal tooth bilobe flexuose enamelled, de- 

 current on both sides, decurrence on remplacing the lamellar tooth 

 behind, no angular knob to it. 



4. SuLCULARiA. Cardinal tooth small striated decurrent before, 



separated by an oblique furrow from a small oblique projection 



[51 

 remplacing the lamellar tooth, with a small fold in it. 



All these shells are transversal and inequilateral ; I have seen 



none yet longitudinal as among the Unios : most of the species are 



ribbed behind. The A. complanata of Say, must form another 



peculiar Genus, which I propose to call Pterosyna ; having the 



united wings behind of the Genus Metaptera. The 2 Alasmodon of 



my monograph belong to the G. Lasmigona. 



102 Sp. Alasmodon ^^onderosum, (Lasm. ponderosa, 1820.) 

 Elliptical, very thick, somewhat swelled, truncate and broadly ribbed 

 behind : yellowish arid laminated outside, white and uneven inside. 

 Length 3-5, diameter 1-3, axis 1-3 of the breadth. 



In lower Ohio and in the Mississippi. — Large heavy shell, five to 

 six inches broad, roughly rugose outside by their concentric lamina. 

 Cardinal tooth nearly trilobe, lamellar angle obtuse, confluent 

 together. 



If these characters of the teeth should separate it from the Las- 

 migona, it may be called Gonamblus. 



103 Sp. Alasmodon rugosum (L. rugosa. 1823.) Shell thick 

 elliptical, hardly swelled, subtruncate behind, broad ribs behind and 

 below, subsinuate below, outside rugose and olivaceous, white and 

 nearly even inside. — Length 3-5, diameter 1-4, axis 3-4 of breadth. 



Tennessee river, rare, 5 to 6 inches broad. Akin to the last, but 

 more flat, less thick and heavy, teeth different, cardinal smaller not 

 trilobe, angular projection less obtuse, with a wrinkle and small tooth 

 at the base. 



104 Sp. Alasmodon sulcatum, (L. Sulcata. 1823.) Shell thick, 

 elliptical and swelled, posterior slope with broad ribs, surface oliva- 

 ceous with large sharp concentric ribs and broad furrows between, 

 inside white incarnate. Length 1-2, diameter 1-3, axis 1-4 of the 

 breadth. 



River Tennessee and Mississippi : fine large shell, 6 inches broad, 

 beautiful nacre ; cardinal tooth crenate, the angular projection acute 

 before, obtuse behind. Beak or apex a little prominent and slightly 

 rugose. Very rare. 



