MYTILUS. 



105 



end to an obtusely rounded edge, gibbous with a gradual slope 

 towards the front and sides, rather thin and glossy : sculpture, 

 concentric and indistinct lines of increase: colour bluish or 

 purple : epidermis dark brown or olive, occasionally yellowish 

 or having a reddish tint, rarely black, microscopically striated 

 lengthwise and shagreened, sometimes puckered in particular 

 spots : margins rather thick, nearly straight on the ventral 

 side and arched behind, obliquely rounded in front : beaks 

 blunt, more or less divergent, placed a little below the termi- 

 nation or point of the shell, which is formed by the dorsal 

 edge of the first-formed part or nucleus ; beneath the beaks 

 is an imperfect lunule or heart-shaped impression, which 

 often appears to be repeated, showing the marks of successive 

 growth: hinge-line nearly straight, occupying almost the 

 upper half of the dorsal margin: ligament very thick and 

 strong : hinge-plate partly folded over the ligament, the middle 

 of which is consequently exposed : hinge furnished with 3-10 

 small tubercular teeth in each valve : inside chalky- white, 

 slightly nacreous at the edges, and pitted (as in Unio margari- 

 tifer) by the impressions of the mantle : muscular scars very 

 distinct. L. 2-3. B. 1-2. 



Yar. 1. ungulata. Shell larger, more pointed at the nar- 

 rower end, and sometimes angular at the other, with the 

 ventral margin incurved : beaks more widely separated than 

 usual. M. ungidatus, Linn. Syst. Nat. p. 1157. 



Yar. 2. incurvata. Shell stunted and bent, often marked 

 by longitudinal ravs of purple. M. incurvatus, Pennant, Brit. 

 Zool. iv. p. Ill, pi. 64. f. 74. 



Yar. 3. Galloprovincialis. Shell broader and natter, with 

 the ventral margin somewhat prominent and the posterior 

 angle more acute than in the typical form : beaks incurved. 

 M. Galloprovincialis, Lamarck, An. sans Yert. vi. p. 126. 



Yar. 4. pellucida. Shell thin, narrower, less gibbous, and 

 beautifully streaked by longitudinal rays of dark brown or 

 purple, which are irregularly disposed. M. pellacidus, Penn. 

 op. c. p. 112, pi. 63. f. 75. 



Habitat : Abundant everywhere and gregarious, from 

 high-water mark to the depth of a few fathoms, on a 

 rocky, stony, or muddy bottom. Var. 1. Cornwall and 

 Channel Isles, at low spring tides. This variety is the 



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