52 University Geological Surrey of Kansas. 



Devonian they reached their highest point in respect to num- 

 bers, about 1400 species ; while at the close of the Paleozoic era 

 they appear to have fallen back to less than 100 species, and to 

 have continued to the present comparatively few in number. 



Before entering upon the description of the Kansas species, it 

 may be well to give an idea of what the animal was and what 

 the markings found on the shells mean. In the first place, there 

 are two shells, or valves, generally unequal in size. The hinge, 

 or the place of union of the two valves, is considered the pos- 

 terior, or hind end, and the opposite edge the front end, while 

 the two sides at right angles to the hinge are the lateral mar- 

 gins or sides. The animals were attached to some foreign object 

 during all or a part of their life, either by a pedicle, a long mus- 

 cular projection from the shell, or by a portion of the shell being 

 cemented to the object. Those having a pedicle usually have 

 an opening in the beak on one of the valves, the pedicle valve, 

 sometimes called the ventral valve, near the hinge, while the 

 others simply have the pedicle extended between the posterior 

 ends of the valves. 



The front portion of the shell is lined by a membrane which 

 divides the cavity into two parts, the anterior or brachial cavity, 

 and the posterior or visceral cavity. In the latter are located the 

 visceral organs and the muscles which open and close the shell 

 and retract and protude the pedicle. The nervous system con- 

 sists of a single ring around the esophagus, in which there are 

 located two ganglia which give off branches to the different 

 organs of the body. The digestive canal consists of but a single 

 convolution, and terminates blindly in the living forms, though 

 it probably communicated with the exterior in certain fossil 

 forms. The pair of muscles which close the shell, the adductors, 

 are large muscles which extend directly from the postero-central 

 part of one valve to the other. By their contraction the shell 

 is closed. The impression left in the shell by these muscles is 

 usually quite prominent. The diductors, or muscles which open 

 the shell, are large muscles attached to the middle of the rear 

 end of the pedicle valve just outside of the adductors, while the 

 other end is attached to a prolongation of the other, brachial, 



