206 



MOLLUSCA. LAMELLIBRANCHIA 



muscles. Occasionally the ligament is preserved in fossil 

 specimens. 



The length of a lamellibranch shell is measured from 

 the anterior to the posterior margin (fig. 92 B, a — p), the 

 breadth or height from the umbo to the ventral margin 

 (d — v) the thickness from one valve to the other at right 

 angles to the lines of length and breadth. 



Fig. 94. Vertical section of the shell of a recent Unio, cut in a radial 

 direction from the umbo ; the right-hand side of the section is near 

 the ventral margin, a, pearly or nacreous layer, in which the later 

 lamella overlap the earlier and extend on to (b) the prismatic layer ; 

 c, periostracum. x 10. 



The shell is secreted by the mantle ; its structure 

 varies in different groups. In some genera it consists of 

 two calcareous layers ; the inner is the pearly or nacreous 

 layer, and is formed of numerous thin lamellae (fig. 94 a) ; 

 the outer is the prismatic layer (figs. 94 b, 95), and is com- 

 posed of prisms placed more or less nearly perpendicular to 

 the surface of the shell, each prism being encased in a 



