THE AMERICAN XA TURAUST. [Vol. XXXV 1 1 . 



bilaterally symmetrica!. What I shall call the distal portion of 

 the dorsal side (Fig. 4) is strongly arched and has a very smooth, 

 beautifully polished surface dotted with minute pores. It is a 

 slaty gray color, becoming a dark chocolate on the left, in that 

 part that was originally deeply imbedded in the matrix. The 

 texture changes rapidly at the anterior end, where there is a 

 broad band of coarse dimples in the polished Moor of a shallow 

 groove, b. p. In front of this groove the plate narrows to form 

 a thick collar, r, very different in appearance from the rest of 

 the plate. It is lighter colored and has a smooth, but irregular 

 surface that lacks the brilliant polish and regularly distributed 

 minute pores of the posterior part. It is provided with a few 

 large openings irregularly distributed. The whole appearance of 

 the collar suggests that it was made to fit into a nearly circular 

 opening and that it was held in place there by stout ligaments 

 attached to its roughened surface. 



A remarkably clear cut, semicircular groove is seen on the 

 beautifully polished and rounded surface of the plate a little way 

 below the band of pores, c. g. It is seen on all the specimens 



lifetime of the animal by a rotatory movement of the plate that 

 brought it in contact with some hard object. The meaning of 

 this groove will be considered farther on. 



somewhat remarkable structure, Fig. 5. ? A small fragment'of 

 the edge of the plate was broken off in excavating the matrix, 

 the outline of the missing portion being indicated by a dotted 

 line. The plate is now seen to be of a uniform dark chocolate 



