THE AMERICAN NATURALIST. [Vol. XXXVII. 



joint from the other. The mode of articulation with the body 

 scales is not shown by any specimens of this species examined. 



In the magnificent specimen originally described by Agassiz, 

 (PI. II, Fig. 6) the animal lies at full length in a natural position. 

 Here the fringing processes are seen edgewise, and have a differ- 

 ent shape and appearance from those of Cephalaspis murchisoni. 

 They hang freely away from the trunk, in a nearly vertical 

 position, with their distal ends bending backwards in graceful 

 curves. Each process has a rounded head that fits into a cup-like 

 depression on the posterior ventral margin of the large dorso- 

 lateral trunk scales. Below the rounded head, the process is at 

 first quite slender, then somewhat abruptly thickened, and 

 finally reduced to a thin lamella with indistinct boundaries. 

 There are from twenty to thirty pairs, beginning just back of 

 the cephalic shield and gradually decreasing in size from that 

 point toward the tail end. The most posterior ones are reduced 

 to mere spines, or rhomboidal plates, loosely articulated to the 

 lateral trunk scales. 



Finally in a small specimen in the Powrie collection at Edin- 

 burgh (No. 139), where the whole animal is seen in outline 

 from below, the fringing processes of both sides are shown 

 folded inwards and flattened against the ventral wall of the 

 body (Fig. 10). On the left side of the figure, one can count 

 about twenty processes. 



The varying appearance of the processes is clue in part to 

 their position and to the way in which they are exposed, and in 

 part to the fact that the plates belonging to different species 

 differ considerably in structure. 



In Cephalaspis murchisoni, the fringe plates are lobed and 

 overlap one another so that their flattened surfaces are directed 

 diagonally forward and outward. In Cephalaspis pagei, they 

 appear to have a similar shape and arrangement, but are armed 

 with coarse projecting spines that give them a decidedly arthro- 

 pod appearance. In Cephalaspis lycllii, the plates lie one 

 directly behind the other without overlapping, while the .promi- 



and the thin, backwardly direc ted distal ends give them a unique 

 and characteristic appearance. 



