CONCHOLOGICAL -WEITINGS. 19 



3. Decurambis. Cardinal tooth bilobe flexuose enamelled, de- 

 .current on both sides, decurrence on remplacing the lamellar tooth 

 behind, no angular knob to it. 



4. SuLCULAEiA. Cardinal tooth small striated decurrent before; 

 separated by an oblique furrow from a small oblique projection 



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 TJnios : 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 ponderosum, (Lasm. ponderosa, 1820.) 

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

 behind : yellowish and 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 Gonamhlus. 



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

 elliptfcal, 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. ft23.) Shell thick, 

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

 ceous with large sharp concentric ribs and broad furrows between, 

 inside white incarnatS. 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. 



