NAYADES. 113 



garitifera, Drapernaud, p. 132, pi. 10, f. 17, 18, 19; Pfeiffer, 

 I, p. 116, pi. 5, f. 11 ; Unio elongata, Lamarck, An. San. Vert., 

 VI, pt. 1st, p. 70; Damaris margaritifera. Leach, MSS., p. 

 10; Mya margaritifera, Miiller, Verm., II, p. 210, No. 396 ; 

 Montagu, Test. Brit., p. 33 ; Donovan, Brit. Sh., pi. 73 ; Pen- 

 nant, Brit. Zool., IV, p. 80, pi. 43, f. 18; Brown, Wernerian 

 Mem., II, p. 505; lb., Ency. Brit., 1st Ed., VI, p. 405 ; lb,, 

 Ency. Edinensis, II, p. 542 ; Alasmodon margaritiferus, 

 Brown, Illust. Conch,, p. 83, pi. 31, f. 1, 2, pi. 32, f. 13, 14, 15, 

 and pi. 30,* f. 1, 2, 3, 4. 



Shell transverse, oblong-ovate ; hinge line considerably arcu- 

 ated ; basal line concave ; umbones flat, rugged, much decor- 

 ticated, and frequently with small, round perforations; an 

 obsolete, gradually widening hollow, extends from the umbones 

 to the base, in both valves ; anterior side very short, and 

 rounded ; posterior side much elongated, and pointed ; surface 

 covered with a brownish-black, strong epidermis ; teeth of the 

 hinge strong, thick, subconic ; right valve with a single tooth, 

 notched on both sides, which locks into a bifurcated, notched 

 tooth in the opposite valve ; muscular impressions near both 

 extremities, and are deep and complicated ; pallial impression 

 very distinctly defined, and deeply marked towards the anterior 

 side ; interior pearlaceous, with irregular streaks of green, and 

 frequently bronzed ; towards the centre are from twenty to 

 thirty irregular, rather deep, nearly round punctures, of the size 

 of the head of a small pin. Length upwards of two inches and 

 a half; breadth five inches and a half. 



Fig. 13, pi. XXI, external view of the teeth. 



Found in the rivers Aun and Avon, Devonshire ; the Con- 

 way; the Irt, Cumberland; the Lune, Lancashire; the Tay and 

 Forth, in Scotland; the Tyrone, Bann, and Slaney, near Ennis- 

 corthy, some of the tributary streams of Lough Neagh, Ireland; 

 and various other localities in Britain and Ireland. 



This shell is familiarly known by the name of the Pearl 

 Muscle. In a Paper on Pearls in the Philosophical Transac- 

 tions, it is mentioned that several pearls of considerable value 

 have been procured from the Alasmodon margaritiferus, 

 which had been found in the rivers Tyrone and Donegal, 

 Ireland. One of them weighed thirty-six carats, and would 

 have been worth £40., but owing to its being impure, it lost 



Q 



