112 NAYADES. 



f. 3; Brown, Illust. Conch., p. 82, pi. 31, f. 3, 3,* 4, .5, 6; lb.. 

 First Ed., pi. 26, f. 3 ; Mya pictorum, Donovan, Brit. Sh., pi. 

 174; Wood, Conch., p. 403, pi. 19» f. Ij 2; My sea JBatava, 

 Turton, Man., p. 20, pi. 2, f. 10. 



Shell inflated, oval ; hinge line subarcuated ; posterior side 

 but very slightly produced ; umbones rounded, placed near to 

 one side, and a little apart from each other ; anterior muscular 

 impressions small, and deep ; pallial impression but slightly 

 defined; inside of a rich pearlaceous lustre; exterior surface of 

 a yellowish-green, frequently beautifully radiated with darker 

 green; lines of growth not deep ; basal line arcuated. Length 

 three-fifths of its breadth ; thickness upwards of two-fifths. 



Figs. 10 and 11, pi. XXI, external and internal view of the 

 teeth. 



This shell is easily distinguished by its more regularly oval 

 form, and smaller size, seldom exceeding an inch and a quarter 

 in length. 



Found in the river Kennet, above Newbury. 



Genus III.— ALASMODON.— Say. 



Shell thick, generally transversely elongate, but variable in 

 form, equivalve, inequilateral; a little gaping posteriorly ; with 

 or without auricles ; umbones for the most part rough, and 

 decorticated, more so anteriorly; hinge with a lamellar, blunted, 

 lateral tooth on the posterior side, situate under the ligament, 

 but destitute of one on the anterior side ; a short, irregularly 

 indented, cardinal tooth in the right valve, which locks between 

 the two irregularly crested teeth in the left valve ; ligament 

 exterior, and much elongated; muscular impressions large, irre- 

 gular, frequently double, and placed near the extremities; pallial 

 impression deeply defined. 



1. Alasmodon mabgaratiferus, pi. XXII, f. 1, 2, 3, 

 and pi. XXI, f. 13. 



Variety 1. Ordinary form, pi. XXIT, f. 1. 



Alasmodon margaritifevum, Fleming, Brit. An., p. 417; Unio 

 margaritiferus, Nilsson, p. 106, No. 2; Turton, Conch. Diet., 

 p. 202, pi. 16, f. 1 ; lb., Man., p. 19, pi- 2, f. 9 ; lb., Brit. Biv., 

 p. 242, pi. 16, f. 1 ; Rossmassler, I, p. 120, pi. 4, f. 72, 73; 

 Thompson, Ann. and Mag. Nat. Hist., VI, p. 55 ; Unio mar- 



