ANATOMY AND PHYSIOLOGY OF ANODONTA FLUVIATILIS. 171 
beneath the epidermis is the prismatic layer (Pl. 4, figs. 4, 6, pr.), 
seemingly composed of solid prisms, but if this portion of the shell is 
placed in dilute acid the interior of each prism is dissolved, leaving 
the walls entire, showing that the layer is made by the deposition of 
calcareous matter in prismatic, generally hexagonal cavities, which are 
themselves formed by the superimposition of fenestrated lamina, 
secreted by the margin of the mantle. (PI. 4, fig. 6, pr.) 
The third or inner layer of the shell is formed by the superimpo- 
sition of very thin diagonal layers, which overlap each other and is 
known as nacre or the nacreous layer. (Pl. 4, figs. 5, 6, nac.) 
The interior of the shell presents an irridescent appearance, which is 
caused by the refraction of the light by the edges of the overlapping 
layers. The prismatic portion being secreted by the edge of the 
mantle extends slightly beyond the nacreous portion. If any thin 
shell is held to the light the prismatic portion can easily be seen 
with the aid of a simple lens. Near the margin of the shell, the 
prismatic portion is the more prominent, while on all other portions 
the nacreous layer is thicker than the prismatic, ae from two to 
four times the thickness. 
The epidermis (Pl. 4, fig. 3), shows no structive: though showing 
under the microscope pigment cells (Pl. 4, fig. 3, p.c.), being with- 
out lime in its composition, it serves to resist the action of carbonic 
acid gas, which is contained in greater or less quantities in all fresh 
water, and which, but for the protection of the epidermis, would de- 
stroy the nacreous and prismatic portions of the shell. On portions 
of the shell, generally the umbo, from being the longest exposed, 
where the epidermis has been worn away, the shell is often much eroded. 
The same conditions occur on other portions of the shell where by acci- 
dent the epidermis has been broken away. Sometimes between the other 
layers the animal secretes a layer of epidermis, thus arresting the 
progress of erosion; layers occurring in this manner are usually colorless. 
The formation of the shell, according to all or nearly all the 
writers on this subject,is not continuous, but occurs at successive 
periods; but this explanation is not entirely satisfactory. The secre- 
tion of the shell is involuntarily performed by the animal and can no 
more be arrested by the will of the animal than the formation of 
‘bone in the human body can be continued or not according to the will. 
There are numerous elevations on the exterior of the shell; in thin 
shells these elevations have corresponding undulations in the interior. 
The shell of the Anodonta being usually very thin, the interior cor- 
responds in form to the exterior. The Anodonta is without the 
teeth of the Unio or Margaritana, but below the ligament there are 
two thickened projections or rudimentary teeth. (PI. 11, fig. 1, r.t.) 
