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THE AMERICAN NATURALIST. [Vol. XXXVI J. 



posterior end, the median dorsal margin forms a rather promi- 

 nent shoulder, s, giving this portion of the appendage a charac- 



The anterior end, which slopes diagonally forward and out- 

 ward, is provided with a beautifully modeled articular condyle, c. 

 It is a smooth, rounded prominence, flattened dorso-ventrally, 

 slightly depressed on its anterior surface, and with a well 

 denned constriction around its base. Jts median end is contin- 

 uous with a distinct ridge that extends along the middle of the 

 median surface of the appendage. Figs. 1-3, r. 



The entire dorsal surface of the joint was covered originally 

 with a polished enamel like layer, similar in color and texture to 

 that on the dorsal shield of Tremataspis. But a large patch of 

 this layer, on that part of the specimen that originally protruded 

 from the matrix, is worn off, thus exposing the hexagonal mesh- 

 work of vessels, filled with a reddish deposit, so characteristic of 

 the middle layer of the shell of Tremataspis. 



The polished outer layer terminates abruptly at the anterior 

 end in a rounded edge that may be followed around the project- 

 ing anterior lateral angle, p. On the dorsal surface, this polished 

 edge presents two well defined curves, a part of the posterior one 

 being destroyed. ( )n the ventral surface, the whole of the enamel 



three well defined rounded incisions, Fig. 1. The resemblance 

 between what is left of the anterior enameled margin on the 

 dorsal side, and the intact margin on the ventral, indicates that 

 both were probably in the uninjured specimen quite similar. 

 The ventral surface of the appendage is perfect in every detail. 



sal surface. It is slightly depressed in the middle, there are 



coarse pits, the mouths of which look towards the distal end of 

 the appendage as though they had at one time been occupied by 



quite different from the smooth suit ace on the dorsal side of the 

 appendage. 



When we look at the appendage from the median side, Fig. 2, 



