76 



MOLLUSCA. 



[Mytilacea. 



concave spines, which increase in size towards the base ; about 

 a third of the shell opposite the liganientary 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 lamina?. 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, pi. XXVI, f. 2. 



Pinna elegans, First Ed., pi. 30, f. 2 ; Leach, MSS., p. 10 ; 

 Pinna papyracea, Turton, Biv., p. 224, pi. 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 11 Dreissena. — Van Beneden. 



Shell boat-shaped, or mytilform ; valves carinated ; ligament 

 internal, except in a little fissure in both valves externally ; 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, pi. XXIX, f. 4, 5, 6, 7, 8, 9- 

 Dreissena polymorpha, Van Beneden, Ann. Sci. Nat., 1835, 

 p. 210, pi. 8, f. 1 to 11 ; Strickland, Mag. Nat. Hist., 1838, p. 

 361; Alder, Mag. Zool. and Bot., II, p. 117; Tichogonia 

 Chemnitzii, Rossmassler, I, p. 113, pi. 3, f. 69; Mytilus poly- 

 morphs, Pallus, Voy. Russ. App., p. 211; Mytilus Wolgce, 

 Chemnitz, XI, p. 205, f. 2028 ; Mytilus Hagenii, Baer, Fur. 



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 very glossy. 



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 Mytilus Linnaeus. 



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. Mytilus edulis, pi. XXVII, f. 11, 12, 13, 14, 15, and 

 pi. XLV, f. 4, 5, 6, 7— First Ed., pi. 29, f. 11, 12, 13, 14, 15. 



Variety 1, pi. XXVII, f. 11. 



Mytilus edulis, Linne, Syst. Nat., p. 1157; Lister, Conch., 

 pi. 362, f. 200; Pennant, IV, p. 110, pi. 63, f. 73; Montagu, 

 p. 159; Donovan, pi. 128; Lamarck, VI, p. 126; Brown, Ency. 

 Brit., VI, p. 423 ; lb., Wernerian Mem., II, p. 515 ; Fleming, 

 p. 411 ; Forbes, p. 43; Tuiton, Brit. Biv., p. 196; Maton and 

 Rackett, p. 105; Mytulus vulgaris, Da Costa, p. 216, pi. 15, 

 f. 5. 



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. Elegans, pi. XXVII, 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. Pellucidus, pi. XXVII, f. 13. 



Mytilus pellucidus, Pennant, IV, p. 1 12, pi. 63, f. 75 ; Chem- 

 nitz, VIII, pi. 84, f. 755; Montagu, p. 160; Donovan, pi. 81 ; 

 Maton and Rackett, p. 107; Brown, Ency. Brit., VI, p. 423; 

 lb., Wernerian Mem., II, p. 415; Turton, Brit. Biv., p. 197, 

 pi. 15, f. 1,2; Forbes, p. 43. 



