882 The American Naturalist. [October, 
placed side by side in corresponding positions on a moistened 
slide. Those pieces in which the fibres are disposed at right 
angles to the rays of light coming in from a window appear, 
to an eye placed at the proper angle to catch the reflected 
light, of a beautiful, glistening, satiny white; while those the 
fibres of which lie parallel to the same rays, and present no 
reflecting surfaces, appear dull and lustreless. These appear- 
anees can be instantly changed from one set to another on 
revolving the slide over an angular distance of 90.° 
A place of natural division, where separation is more readily 
effected than elsewhere occurs between the 3d and 4th inner 
layers. The three innermost ones then appear to constitute 
together the membrana putaminis. There is apparently 
no disposition toward the formation of a lenticular air-space ; 
this being prevented by the high elasticity of the membranous 
shell, which causes it to continually contract over the con- 
tents as they shrink through evaporation. This contractility 
appears, however, to be limited, as noted above. Perhaps it 
is an adaptation to the varying conditions of moisture and 
drought to which these eggs, and others of their kind, are sub- 
jected. 
The external six or seven layers are more closely bound 
together, and constitute the shell proper. The depressions in 
the surface of one layer resulting from the wavy course of the 
fibres, accommodate elevations on the surface of adjacent ones, 
thus greatly increasing the strength of the shell and making 
possible that partial invasion of one layer by the fibres of 
adjacent ones which is mentioned above, and which becomes 
more marked externally. These binding fibres may be seen 
to stretch and break as two layers are torn asunder. 
The wavy structure also produces on the surface beneath 
the calcareous crust, and especially where two shells have been 
united, a peculiar pebbled appearance, like the pebbled bind- 
ing of books. The outer layers in which the calcareous matter 
is deposited, have a less regular structure—the fibres being 
more irregularly interwoven; but when the mineral matter is 
removed with acid, the fibres readily disentangle and separate, 
being bound by no other cementing substance. Cement sub- 
