132 BULLETIN OF THE BUREAU OF FISHERIES. 



the shell extends backward in an almost straight line. In specimens of mussels growing 

 on wharf piles in protected situations the ventral edge sometimes presents a slightly 

 convex outline, while, on the other hand, mussels growing on the rocks or mud where 

 they are subjected to swift currents and exposure often show a decidedly concave 

 under surface. 



The size of the adult mussel varies from 2 to 4 inches in length, from 1 to 2 inches 

 in height, and from %" to 2 inches in breadth. Occasionally specimens 4^ inches long are 

 found. The proportions of length to height and breadth vary with the age of the mussel. 

 Individuals less than a year old show a length, breadth, and height which are to each 

 other as 2.75 : 1.5 : 1; while in older ones they are to each other about as 2.25 : 1.15 : 1, 

 indicating that in adults growth in breadth is proportionately more rapid than in length. 



The color of the shell varies from violet or blue-black to a pale blue. When dried it 

 takes on a brownish hue. This change of appearance is due to the hornlike covering 

 of the shell, the periostraeum, which is itself brown. The characteristic violet color of 

 the shell comes from the thick prismatic layer which lies immediately below the thin 

 periostraeum and which contains a deep blue pigment. The general hue of the shell is 

 therefore due to a combination of the brownish, transparent periostraeum and the under- 

 lying layer of deep blue calcareous matter. This results in a variety of color variations 

 according to the thickness and density of the periostraeum and the amount and distrib- 

 tion of the pigment in the prismatic layer of the shell. The arrangement of the pigment 

 is in bands which run from the umbo in a radiating manner to the posterior end of the 

 shell. Most commonly the stripes lie so closely together that it gives the shell surface 

 a uniform dark blue color. Sometimes, however, the bands of color are few in number 

 or entirely absent. In the latter case the mussel is colored a uniform brown or yellow- 

 brown by the periostraeum, while in the former case it is marked with alternate blue and 

 brown bands which radiate from the umbo to the posterior edge of the valve. 



The inner surface of the shell is divided sharply into two regions, an inner, glossy 

 white or pinkish-white mother-of-pearl layer and an outer deep blue border about three- 

 sixteenths of an inch wide. The line of demarcation between these two layers is sharp 

 and may be either straight or serrated in outline. The blue layer is absent in the hinge 

 region. 



Six impressions which mark the attachment of muscles are conspicuous on the inner 

 white surface: (1) The largest and most prominent is more or less circular in form and 

 located posteriorly near the dorsal border. It marks the point of attachment of the 

 posterior adductor muscle (fig. 104, PAD, opp. p. 132). (2) Running anteriorly from 

 the dorsal edge of .this impression is another, linear in form (fig. 104, PRet), which marks 

 the point of attachment of the posterior retractor muscles of the foot and byssus. (3) 

 A third, somewhat triangular impression lying just posterior to the impression of the 

 posterior adductor muscle marks the insertion of the muscles of the anal membrane 

 (fig. 104, An). (4) At the anterior end of the shell on the ventral border is an impression 

 where the anterior adductor muscle is attached (fig. 104, A Ad). (5) Just above it on 

 the dorsal edge is another which marks the point of insertion of the anterior retractor 

 muscles of the foot and byssus (fig. 104, A Ret). (6) A long, narrow, linear impression 

 extending along the lower edge of the shell from the impression of the anterior adductor 

 muscle to that of the posterior adductor muscle and just inside the border of the nacreous 

 layer forms the line of attachment for the pallial muscles (fig. 104, Pal). 



