Anatomy and Physiology of Akodokta 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 ts 

 placed m 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 laminae, 

 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, generally from two to 

 four times the thickness. 



The epidermis (PL 4, fig. 3), shows no structure, 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 t)f 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. (PL 11, fig. 1, r.t.) 



