364 MOLLUSC A. 



to the dorsal knob or umbo the oldest part of the shell 

 and the umbo generally points towards the anterior end. 

 The greenish-brown soft (" horny ") layer of the shell is 

 often worn away near the umbo on each side, and then 

 displays the median " prismatic " layer of lime. Internally 

 there is a pearly layer. Lines of growth on the shell mark 

 the position of the margin in former years, the youngest 

 part being obviously at the edge. 



The shell is a cuticular structure, i.e., it is made by the 

 epidermis of the mantle. ! It consists mostly of calcium 

 carbonate plus an organic substance called conchiolin. 



Internal Appearance. 



When the right half of the shell is folded back, the 

 anterior and posterior closing muscles being carefully cut 

 close to the gently raised valve, the mantle folds are seen 

 lining the shell and forming posteriorly the ventral inhalent 

 and dorsal exhalent lips. The ventral lips have papillary 

 processes. (In some bivalves, e.g., My a, the lips of the 

 mantle are prolonged into retractile siphons.) Internal to 

 the mantle two gill plates lie on each side ; projecting from 

 between these is the foot, muscular ventrally, softer dorsally ; 

 the median dorsal pericardium is just beneath the ligament ; 

 the ventricle shines through its walls, and the dark-coloured 

 kidneys are seen through its floor. Below the anterior 

 closing (adductor) muscle, the large mouth will be found, 

 bordered beneath by two lip processes (labial palps) on each 

 side; above the posterior closing muscle the food canal 

 ends. The whole space between the two mantle flaps is 

 called the mantle cavity, and it is divided by a slight parti- 

 tion at the bases of the gills into a large ventral infra- 

 branchial chamber, and a small dorsal supra-branchial 

 chamber which ends at the exhalent orifice. 



On the valve of the shell folded back, are seen a number 

 of concentric (pallial) lines, marking the gradual extension of 

 the mantle and the consequent growth of the shell. A few 

 small pearls may also be seen; they are formed by the 

 enclosure of some minute grains of sand in the prismatic 

 layer. The following muscles are inserted on the shell and 

 leave impressions : 



