MVTILACKA.] 



MOLLUSCA. 



Generally rather shorter and broader than the former two 

 varieties, the anterior side less indented than the edulis, and 

 much thinner and more transparent, both in the shell and epi- 

 dermis, which is generally of a rich yellow, or amber, under 

 which the shell is beautifully radiated with deep purple, or 

 blue ; inside very smooth and glossy, with the radiations very 

 distinct ; the crenulations under the beaks small. 



This variety is much less common than the M. edulis, and is 

 generally met with in the estuary of rivers. 



Variety 4. Subsaxatilis, pi. XLV, f. 4, 5, 6, 7. 



Mytilus subsaxatilis, Williamson, Mag. Nat. Hist., VII, p. 

 353, f. 48, a, b, c, d ; Mytilus angulatus, Alder ; Mytilus soli- 

 tarius, Mark ; Mytilus edulis, var. crassus, Brown, MSS. 



Shell strong, solid, very thick in proportion to its length ; 

 beaks considerably elongated ; hinge line straight, which is par- 

 ticularly obvious in the young shell (f. 6); front seam, or union 

 of the valves, undulous ; colour of a grayish-blue ; the epider- 

 mis dull olive ; young shells deep bistre-brown. 



Inhabits the coasts of Yorkshire, Durham, and Northumber- 

 land. 



Mr. Williamson says this shell " is found in large pools left 

 by the retiring tide, in groups of three or four together, firmly 

 attached by their strong byssus to the under surface of large 

 stones." He considers it a distinct species, and says " the cha- 

 racter of its habitat, combined with its peculiar solid form, give 

 it as good a title to be styled a distinct species as, if not a 

 better one than, that of the transparent shell of the M.pellu- 

 eidus, or the small blunted form of the shell of M. incurvatus, 

 does these species respectively." 



I first noticed this variety, in 1810, in company with my late 

 valued friend Mr. Hancock, of Newcastle, at the mouth of the 

 Tyne, below Tynemouth, adhering to stones left dry by the 

 tide, in small groups ; and recorded it as a mere variety of 

 M. edulis; not being able to detect any specific distinction 

 in the animals. There can be but little doubt that the five 

 forms enumerated are only permanent varieties of the same 

 species. 



Variety 5. Incurvatus, pi. XXVII, f. 12. 



Mytilus incurvatus, Pennant, IV, p. Ill, pi. 64, f. 74; Mon- 

 tagu, p. 160; Maton and Rackett, p. 106, pi. 3, f . 7 ; Brown, 

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

 Turton, Biv., p. 197 ; Fleming, Edin. Ency., VII, p. 99- 



Shell strong, opaque, broader at the base, in proportion to 

 its length, than any of the preceding varieties ; posterior side 

 much incurvated; shell for the most part greatly inferior in size 

 to the other varieties, although in some few instances we have 

 found them an inch and a half in length. 



2. Mytilus crenatus, pi. XXIII, f. 1, 2. 



Mytilus crenatus, First Ed., pi. 31, f. 1, 2; Davies and 

 Willcox, Ann. of Phil., August, 1825, p. 148; lb., Zool. 

 Journ., I, p. 584 ; Lamarck, VI, p. 120 ; Ency. Meth., pi. 217, 

 f. 3. 



Shell oblong-ovate, subtrigonal, beaks rather obtuse ; with 

 strong, longitudinal, rounded ribs, producing a crenulated mar- 

 gin ; slightly wrinkled transversely; covered with an olive 

 epidermis, beneath which the shell is of a reddish-violet colour; 

 inside tinged with golden-yellow, transversely streaked with 

 purple, bluish at the margins, and crenulated. Length nearly 

 three inches ; breadth an inch and a quarter. 



This is an introduced species, having been brought from 

 India, by the ship " Wellesley," in 1816; and has propagated in 

 Portsmouth Harbour, where it is now completely naturalized. 



Genus 13. — Modiola Lamarck. 



Shell subtransverse, equivalve, regular, oblique; form oblong, 

 somewhat wedge-shaped, and greatly inequilateral; anterior side 

 very small, and obtuse ; posterior side rounded, and close ; an- 

 terior margin slightly gaping, for the passage of the byssus, and 

 forming, with the base, a line oblique to the dorsal one ; beaks 

 nearly lateral ; outside covered with a strong, horny epidermis ; 

 hinge without teeth ; ligament elongated, and subinternal ; two 

 muscular impressions, the posterior one large, sublateral, elon- 

 gated, and irregular, the anterior one small, and terminal ; the 

 pallial impression irregular, and destitute of a sinus. 



1. Modiola papuana, pi. XXVII, f. 1 , 2, 3, 4, 5, 6. 



3Iodiola papuana, First Ed., pi. 29, f. 1, 2, 3, 4, 5, 6; La- 

 marck, VI, p. Ill ; Leach, MSS., p. 11; Modiola vulgaris, 

 Fleming, p. 412; Modiola modiolus, Turton, Biv., p. 199, pi. 

 15, f. 3, young; Mytilus modiolus, Pennant, IV, p. 113, pi. 66, 

 f. 77 ; Montagu, p. 163 ; Donovan, pi. 23. 



The young shell, pi. XXVII, f. 2, 5, 6. 



Mytilus curtus, Pennant, IV, p. 112, pi. 64, f. 76 a, young; 

 Mytilus barbatus, Donovan, pi. 70; Montagu, p. 161. 



Shell strong, convex, rounded and prominent, transversely 

 subovate, oblong; posterior side, or that close to the beak, 

 extremely short, and slightly rounded ; anterior side extremely 

 long, and much rounded ; basal line slightly concave ; outside 

 covered with a strong, thick, purplish-black or dark brown epi- 

 dermis, and longitudinally wrinkled ; inside smooth, and white, 

 exhibiting pearlaceous reflections in some specimens. Length 

 at broadest side three inches ; breadth six inches. 



The fine specimen from which our drawing was made, was 

 dredged in the British Channel, and is in the cabinet of Wm. 

 Nicol, Esq., Edinburgh ; and I possess a specimen, measuring 

 seven inches in length, and three inches and a quarter in 

 breadth, which was caught by a fisherman's line, near the Bell 

 Rock, coast of Forfarshire. 



The young shell in its first stages is frequently of a pale 

 yellowish-white, beautifully radiated with pink or chestnut- 

 brown, with transverse streaks of the same colours ; as exhi- 

 bited in our figures 5 and 6. In a more advanced stage, it is 

 covered with a strong, chestnut epidermis, having the anterior 

 side and base beset with long, unequal, straggling filamentary 

 processes, which have been termed a beard. This beard is 

 merely a prolongation of the epidermis, which drops off when 

 the shell grows to about two inches. In this condition it is the 

 M. barbatus of authors. 



Variety 1. Umbilicata, pi. XXVII, f. 3, 4. 



Mytilus umbilicatus, Pennant, p. 228, pi. 68 ; Donovan, pi. 

 40; Montagu, p. 164. 



Shell with a deep, intorted, and wrinkled depression under 

 the beak. 



This is merely an accidental, and by no means uncommon, 

 variety of the shell. 



The M. papuana is a deep water species, and is firmly fixed 

 to the bottom by a byssus, composed of long silky filaments. 



