GENERAL ANATOMY OF A LAMELLIBRANCH. 271 



Explanation of letters for Fig. 142 : 



A. Anterior end. P. Posterior end. D. Dorsal surface. V. Ventral 

 surface, a. Mantle, ab. Abdomen. 6. Foot. br. Branchial siphon. 

 cl. Cloacal siphon, d. Anterior adductor muscle, e. Posterior adductor 

 muscle. /. Posterior foot-retractor muscle, h. Anterior foot-retractor 

 muscle, hi. Hinge ligament. i<j. Inner gill of left side. Ip. Labial 

 palpi, mo. Mouth, oy. Outer gill of left side. s. Eight valve of shell. 

 u. Uinbo. 



I. Examine first the outer surface of the shell and 

 notice : 



a. The two elongated irregularly oval pieces or valves 

 of the bivalve shell, one on each side of the body. Notice 

 the thin layer of horny-brown, or red or olive-green epider- 

 mis, which covers those parts of the shell which have not 

 been eroded. 



b. On the surface of each valve the concentric lines of 

 growth. 



c. Near the middle of one side of each valve, an area, 

 usually much eroded, around which the successive lines 

 of growth are arranged ; this, the oldest part of the shell, 

 is the umbo or beak (Fig. 142, u) and the margin upon 

 which it is situated the dorsal margin (Fig. 142, D). 



d. Notice that the ventral margins of the valves, and 

 their ends, are free from each other, as well as the greater 

 portion of their dorsal margins. 



e. Near the umbones they are united by the brown 

 horny hinge-ligament (Fig. 142, hi). 



f. The ligament is posterior to the umbones, and thus 

 furnishes a means of distinguishing the somewhat pointed 

 posterior end of the shell (P) from the more regularly 

 rounded anterior end (A). 



</. AVutch a living mussel in the water, and notice the 

 soft white margins of the mantle which protrude slightly 

 beyond the edges of the shell. 



