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370 TRANSACTIONS OF THE CANADIAN INSTITUTE. [Vou. VIII. 
the adambulacral ossicles are exposed. The plates of the right hand 
side are even more reduced by weathering. 
The dorsal side of the arm was protected by two rows of very thin 
plates, placed directly over the ambulacral ossicles. They are oblong in | 
shape and curve outwards to meet the side plates. Owing to their ex- 
treme thinness and to the fact that they are firmly attached to the 
ambulacral ossicles by secondary calcification, their detection was a 
matter of some difficulty. That they exist as separate elements is 
proved by the discovery, in a thin section, of a distinct suture between 
them ; that they are not really part of the ossicle is proved by their 
different microscopic appearance. I am not prepared to state that these 
dorsal plates were originally calcareous ; their microscopic,examination 
shows a different aspect from the other parts, and the calcite of which they 
are now composed is evidently secondary. This point is of importance in 
view of the fact that Dr. Stiirtz is of the opinion that many of these ancient 
forms were destitute of a-dorsal system of shields on the arms. The 
dorsal aspect of the ambulacral ossicles is very different from the ventral. 
Along the mesial line they present a slightly concave inner edge ; as they 
are strictly alternate, the two adjacent angles of a contiguous pair on one 
side fit into the concavity of the one opposite. Passing outwards the 
anterior and posterior margins converge and curve forward to the distal 
border of the side plate. At the extremity, the ossicles are deeply 
notched so as to present a forked appearance. Each prong of the fork 
is no thicker than a hair at its end. The inner portion of the ossicle is 
extended downwards to form the main part of the ossicle as viewed 
ventrally ; the “toe” of the “boot” is a ventral expansion connected 
with the posterior prong of the fork, which continues outward along the 
distal margin of the adambulacral plate. The anterior prong borders 
the ambulacral pore distally. Figure 3 shows a section across two 
ossicles. The round opening is the ambulacral pore, which seems to pass 
through the ossicle to make connection with the ambulacral groove. 
Figure 6 shows the dorsal aspect ofan arm. The cover plates are closely 
attached to the ambulacral ossicles ; distally they are almost intact, but 
proximally they are much worn away exposing the two wing-like ex- 
tensions of the ossicles. 
The dorsal side of the disc shows an elegant pentagonal rosette, 
composed of twenty plates (Figure 1). Ten of these plates radiate from the 
centre in pairs and occupy the interradial positions ; they are probably 
analogous with the dorsal shields of better known forms. From the 
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