76 MOLLUSCA. 
concave spines, which increase in size towards the base; about 
a third of the shell opposite the ligamentary side is destitute of 
ribs, but furnished with fine, oblique striae, which extend from 
the margin of the valves till they meet the ribs; hinge side 
nearly straight, but inclining slightly towards the beaks, and 
sometimes a little convex in the middle; base gaping, somewhat 
rounded, and tapering towards the apex; hinge side a little 
longer than the other; inside smooth, somewhat dull nacred ; 
under the apex a few transverse, cartilaginous lamine. Length 
six inches; breadth at the base somewhat more than two and a 
half. 
Found in deep water off the Eddystone Lighthouse, near 
Plymouth, and also on the Dorsetshire coast, near Weymouth; 
at Torbay, and on the Sussex coast. 
3. Pinna ELEGANS, pl. XXVI, f. 2. 
Pinna elegans, First Ed., pl. 30, f. 2; Leach, MSS., p. 105 
Pinna papyracea, Turton, Biy., p. 224, pl. 20, f. 3. 
Shell thin, elongated, of a yellowish horn-colour ; provided 
with from six to nine inequidistant, smooth, rounded ribs, 
emanating from the apex, and radiating towards the base, 
occupying nearly the entire surface of the valves; narrow end 
inflated for some distance beyond the middle, but much com- 
pressed towards the base, which is rounded. Length six inches 
and a half; breadth at base nearly three inches. 
A deep water species, dredged by Mr. Prideaux on the 
Devonshire coast, and has been found off Torbay. 
Genus 1]1.—Dreiss—ENA.—Van Beneden. 
Shell boat-shaped, or mytilform; valves carinated; ligament 
internal, except in a little fissure in both valves externally 5 a 
rude cardinal tooth is situate under the umbo in the right 
valve, which locks into a corresponding cavity in the left valve ; 
in the umbonal angle of both valves is placed a transverse par- 
tition, for sustaining a closing muscle; anterior marginal seam 
with a fissure near its centre, for the passage of the byssus. 
1. DreissENA PoLyMorPHA, pl. XXIX, f. 4, 5, 6, 7, 8, 9. 
Dreissena polymorpha, Van Beneden, Ann. Sci. Nat., 1835, 
p- 210, pl. 8, f. 1 to 11; Strickland, Mag. Nat. Hist., 1838, p. 
361; Alder, Mag. Zool. and Bot., I, p. 117; Tichogonia 
Chemnitzii, Rossmassler, I, p. 113, pl. 3, £69; Mytilus poly- 
morphus, Pallus, Voy. Russ. App. p- 211; Mytilus Wolge, 
Chemnitz, XI, p. 205, f. 2028; Mytilus Hagenii, Baer, Fér. 
Bull. Sci. Nat., 1826, p. 140; —— ? Volgensis, Gray, Ann. 
Phil., 1825. 
Shell cymbiform, extremely flat in front, and acutely carinated 
behind; umbones incurved, pointing anteriorly, turned down- 
wards at the points, and very closely pressed together; anterior 
side with an elongated, cordiform groove, reaching nearly to 
the centre of the valves, towards the base of which is a large, 
wide, elongated hiatus, acute at both ends, always situate in 
the right valve, for the passage of the lyssus; the edge of the 
opposite valve being always entire; external surface covered 
with a rather strong, olivaceous, irregularly wrinkled epidermis, 
with indications of nearly obsolete, longitudinal, divergent ribs, 
or elevations; beneath this the shell is beautifully mottled with 
zigzag brown, or purple, transverse clouds or markings ; inside 
dull white, with the pallial and muscular impressions of a purple 
hue, and yery glossy. 
[Myrinacra. 
This species is subject to great variety in length and breadth, 
varying from an inch and five-eighths to an inch in length. 
Some are elongated and narrow; others short and broad. It 
inhabits the commercial docks in various parts of Great Britain, 
and is to be met with in most of our canals. It is common in 
the Bridgewater canal, and also in many places in the canal 
betwixt Manchester and Hull; and was found by my friend 
Mr. Stark in the Union canal, near Edinburgh, in 1834. The 
animals are gregarious, generally found adhering in clusters by 
a strong byssus. 
This shell is not an aboriginal of Britain, being a native of 
the Volga, the Danube, and other continental rivers, and has 
been introduced upon timber; but is now so generally diffused 
throughout Britain, that it may fairly be considered as a British 
shell. 
Genus 12.—Myritus.—Linneus. 
Shell equivalve, regular, longitudinal, somewhat wedge- 
shaped, with the beaks terminating in a pointed summit ; pos- 
terior side rounded, and closed; base forming a continuous line 
with the anterior margin, in a direction oblique to the hinge 
line; anterior margin gaping slightly in the centre, for the pas- 
sage of the byssus; hinge destitute of teeth; ligament marginal, 
greatly elongated, and subinternal; outside covered with a 
strong horny epidermis; two muscular impressions, the poste- 
rior one large and irregular, the anterior very small and termi- 
nal; pallial impression irregular, destitute of a sinus. 
1. Myritus EpuLIs, pl. XXVII, f. 11, 12, 13, 14, 15, and 
pl. XLV, f. 4, 5, 6, 7.—First Ed., pl. 29, f. 11, 12, 13, 14, 15. 
Variety 1, pl. XX VII, f. 11. 
Mytilus edulis, Linné, Syst. Nat., p. 11573; Lister, Conch., 
pl. 362, f. 200; Pennant, IV, p. 110, pl. 63, f. 73; Montagu, 
p- 159; Donovan, pl. 128; Lamarck, VI, p. 126; Brown, Ency. 
Brit.. VI, p. 423; Ib., Wernerian Mem., II, p. 515; Fleming, 
p- 411; Forbes, p. 43; Turton, Brit. Biv. p. 196; Maton and 
Rackett, p. 105; Mytulus vulgaris, Da Costa, p. 216, pl. 15, 
8 bin 
Shell longitudinal, rather strong, oblong; beaks pointed, sub- 
acute; anterior side angulated, nearly parallel; posterior side 
produced ; covered with a thick, smooth, dark olive-green or 
brownish epidermis, beneath which the shell is white or blue, 
longitudinally radiated with deep blue or purple; inside white, 
its margin blue. Length three inches; breadth generally about 
half its length; but varying in this respect in different localities. 
Common on most of the British coasts, adhering to rocks, 
stones, and gravel, by a filamentary byssus. 
Variety 2. Exncans, pl. XX VI, f. 14, 15. 
Mytilus elegans, Leach, MSS., p. 11. 
More elongated than the former variety, with the posterior 
side less rounded; the epidermis very glossy. 
Found in the mouth of the river Tay, below Broughty 
Ferry, in deep water. 
Variety 3. Priucipus, pl. XXVII, f. 13. 
Mytilus pellucidus, Pennant, IV, p. 112, pl. 63, f. 75; Chem- 
nitz, VIII, pl. 84, f. 755; Montagu, p. 160; Donovan, pl. 81; 
Maton and Rackett, p. 107; Brown, Ency. Brit., VI, p. 423; 
Ib., Wernerian Mem., II, p. 415; Turton, Brit. Biv. p. 197, 
pl. 15, f. 1, 2; Forbes, p. 43. 
