Bkkde.] Carboniferous Invertebrates. 109 



pelecypod, let us turn our attention to the shell. The hard 

 parts of animals are the only ones preserved in the rocks. In 

 the clams the shell only is found, and the impressions on the 

 inside of it are all we have to judge from as to the exact nature 

 of the animal which inhabited it ; consequently, it is of great 

 importance to know what these markings are and what they 

 represent. 



The elevated portions of the shell, which project upward near 

 the hinge, or where the two halves are fastened together, are 

 called the beaks or urabones. These generally point toward 

 the front of the shell. If we hold the shell so that the beaks 

 point away from us, the half on the right side will be the right 

 valve and the other half the left valve. The edge of the shell 

 on which the beaks are situated is the dorsal edge or portion of 

 the shell. The side opposite ( below) this is called the ventral 

 portion, and its edge is called the ventral or pallia! margin. 

 The line where the two halves are united is called the hinge 

 line. The hard, gristly material that forms the hinge is called 

 the ligament. The projections along the hinge line, which lit 

 into the corresponding sockets in the opposite valve, are called 

 the teeth. We have previously mentioned the adductor scars, 

 the pallial line, and the notch in the pallial line caused by the 

 muscle (retractor) which pulls in the siphon. 



The pelecypods described in the following page were inhabi- 

 tants of salt water. 



I wish to acknowledge the kind aid in this work given by Mr. 

 Austin F. Rogers. He has also found and worked out several 

 species in the following list since the work closed. For full 

 bibliography the reader is referred to Weller, Bull. U. S. (Jeol. 

 Surv. No. 153, (1898). 



