Nayades.] MOLLUSCA. 



a, b; Drapernaud, pi. 11, f . 3 ; Mya piciorum, Donovan, Brit. 

 Sh., pi. 174; Wood, Conch., p. 403, pi. 19, f. 1, 2; Mysca 

 Batava, 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. 4 and 5, pi. XXXI, 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 3 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 margaratiferus, pi. XXXI, f. 1, 2; pi. 

 XXXII, f. 13, 14, 15; and pi. XXX,* f. 1, 2, 3, 4. 



Variety 1. Ordinary form, pi. XXXI, f. 1, 2. 



Alasmodon margaritiferum, Fleming, Brit. An., p. 4 17; 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- 

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

 I, p. 1 16, pi. 5, f. 11 ; Unio elongata, Lamarck, An. San. Vert., 

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

 10; Mi/a margaritifera, Muller, 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. 



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 



83 



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. 



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

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

 Forth, in Scotland; the Tyrone, Bann, and Slaney, 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 have 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 

 much of its value. Other pearls, from the same places, have 

 sold for from £4. 10s. to £10. One at the last named price 

 was bought by Lady Glenlealy, who had it placed in a necklace, 

 and refused £80. offered for it by the Duchess of Ormond. We 

 are told by Camden, that Sir John Hawkins had a patent for 

 fishing pearls in the river Irt, Cumberland. There was also a 

 great fishery for pearls in the river Tay, Scotland, which ex- 

 tended from Perth to Loch Tay; and, it is said, the pearls sent 

 from thence to London, from the year 1761 to 1764, were 

 worth £10,000. sterling. It is not uncommon at the present 

 time to find pearls in the Teith and Tay, worth from £1. to 

 £2. each. 



Variety 2. Roissyi, pi. XXXII, f. 1.3, 14, 15, and pi. 

 XXX,* f. 4. 



Unio Roissyi, Michaud, Compl., p. 112, pi. 16, f. 28; 

 Proceedings Brit. Assoc, for 1838; Unio margaritifer, Ross- 

 massler, I, pi. 4, f. 74 ; Forbes, Mai. Mon., p. 44 ; Fleming, 

 p. 417 ; Thompson, Ann. and Mag. Nat. Hist., VI, p. 56. 



This variety differs from the common form in the following 

 particulars. The shell is much longer, in proportion to its 

 breadth ; it is more uniformly ovate, the teeth somewhat more 

 elevated; the exterior covered with very fine, transverse stria;, 

 the epidermis blacker, and general surface more smooth ; the 

 hinge and basal lines less arcuated, interior more of a reddish- 

 blush, or flesh-colour, and the shell considerably thinner than in 

 the ordinary form ; the muscular impressions are smaller, and 

 less deep ; the punctures are situate nearer the umbones, are 

 elongated, and not deeply sunk, being more lacrymose in their 

 appearance. 



Figs. 13, 14, and 15, pi. XXXII, are the Isle of Man variety, 

 and f. 4, pi. XXX,* is from the Teith. 



This form inhabits the Teith, at Callander, Perthshire. 

 Forbes has found this elegant variety in the Black river, near 

 Kirk Bradden Church, and at Castleton, Isle of Man, where it 

 is common. He says " it was formerly much sought after by 

 the inhabitants for the sake of the pearls, which it not unfre- 

 quently contains." Mr. Thompson says this variety is common 

 to several localities in Ireland. 



Variety 3. Minor, pi. XXX,* f. 2. 



Unio margaritifera, minor, Rossmassler, II, p. 21, pi. 9, f. 

 129; Nat Michaud. 



Shell small, subcompressed ; valves very thick ; the cardinal 

 teeth compressed, and small; general form like variety 1. Sel- 

 dom exceeding an inch and a half in length ; and three inches 

 and a half in breadth. 



