12 LAND AND FRESH-WATER MOLLUSKS. 
consisting of delicate prismatic cells of calcareous 
matter, and an internal layer, which is shining and 
pearly, and makes up nearly the whole thickness 
of the shell. This latter, or nacreous layer, con- 
sists of folded plates of carbonate of lime, which, 
by refracting the light, give rise to that charac- 
teristic pearly lustre of the interior: this por- 
tion, when polished, forms ‘‘ mother-of-pearl.” 
For the microscopic examination of the shell 
structure, sections are necessary ; but in place of 
these, thin edges of broken portions of the shell 
may be employed. The shell grows partly by 
addition to the margin and partly to the interior. 
In the interior of the shell, the following 
markings and parts are to be distinguished :— 
The umbonal cavity corresponding to the umbo. 
The hinge-line of the right valve presents a 
prominence or tooth (Plate II., fig. 2) towards the 
front, which fits into a depression between two 
teeth in the left valve. On the posterior part of 
the hinge-line of each valve is developed an 
elongated tooth: these are said to be lateral; 
but when they are situated beneath the umbo, as 
in the fresh-water cyclas, they are termed cardi- 
nal. On the inner surface of the valves are seen 
two impressions or distinct pits, one near the 
posterior (a, a’), and one near the anterior sides 
in each valve, made by two strong muscles 
extending internally from one valve to the other. 
