GENERAL ANATOMY OF A LAMELLIBRANCH. 273 



ductor is a mucli smaller scar, the point of attachment of the 

 posterior retractor muscles of the foot, and near the scar of 

 the anterior adductor are two small marks, indicating the 

 points where the anterior foot retractors and the foot pro- 

 tractors are attached. 



d. In the shell of Unio, notice the hinge teeth, which 

 are situated upon the thickened dorsal edges of the 

 valves. 



1 . The anterior or cardinal teeth are situated under the 

 umbones, and are very irregular in shape. In most 

 species there is one in the right and two in the left valve. 



2. Back of these and under the hinge ligament are the 

 long, narrow, lateral teeth; one in the right and two in the 

 left valve. 



e. The shell is made up of three layers. 



1. The horny epidermis which covers the outer surface 

 of the valves, and is reflected over their free edges into 

 the mantle. 



This layer is directly continuous with the hinge liga- 

 ment. 



2. The pearly layer which covers the greater portion of 

 the inner surface. When carefully examined with a lens 

 this layer is seen to be made up of very thin, flat, super- 

 imposed layers of pearl, the edges of which are visible 

 a.s line sinuous lines on the inner surface. 



3. The darker prismatic layer is between these two, 

 and is exposed only along the edge of the inner surface. 

 When examined with a lens it is seen to be made up of 

 polygonal prisms, placed perpendicular to the surface of 

 the shell. 



/. Examine a fractured surface with a hand-lens, and 

 notice these layers, as seen in section. 



III. In order to expose the body, raise one valve of the 



