MOLLUSCA. 
83 
Nayades.] 
a, b; Drapernaud, pi. 11, f. 3 ; Mya pictorum, 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. 417; iTnio 
mar garitif eras, Nilsson, p. 106, No. 2; Turton, Conch. Diet., 
p. 202, pi. 16, f. 1 ; Ib., Man., p. 19, ph 2, f. 9 ; Ib., 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. 116, pi. 5, f. 11; Unio elongata, Lamarck, An. San. Vert., 
VI, pt. 1st, p. 70; Damaris margamtifera, Leach, MSS., p. 
10; Mya margaritifera^ Muller, Verm., II, p. 210, No. 396; 
Montagu, Test. Brit., p. 33 ; Donovan, Brit. Sh., pi. 73 ; Pen- 
nant, Brit. Zook, IV, p. 80, pi. 43, f. 18; Brown, Wernerian 
Mem., II, p. 505 ; Ib., Ency. Brit., 1st Ed., VI, p. 405 ; Ib., 
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, aud 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. 
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, 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 2v Roissyi, pi. XXXII, f. 13, 14, 15, and ph 
XXX,* f. 4. 
Unio Roissyi, Michaud, Comph, p. 112, pi. 16, f. 28; 
Proceedings Brit. Assoc, for 1838; Unio margaritifer, Ross- 
massler, I, ph 4, f. 74 ; Forbes, Mah 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 striae, 
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, ph XXXII, are the Isle of Man variety, 
and f. 4, ph XXX,* is from the Teith. 
This form inhabits the Teith, at Callander, Perthshirei 
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, ph XXX,* f. 2. 
Unio margaritifera, minor, Rossmassler, H, p. 21, ph 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. 
