CANADIAN FOSSILS. 87 



surfaces. The two marginal excavations of each plate are separated 

 by a thin wall-like partition, and bordered at the sides and outer edge 

 of the plate by a sharp lim. It thus follows that the channel around 

 the margin of the animal was divided into twice as many compart- 

 ments as there are marginal plates. The integument of the upper 

 side, supposed to be the side on which the marginal plates are 

 excavated, is connected to the inner edge of those plates, and does 

 not extend over them ; its surface is roughened by small tubercles. 

 The integuments of the under side appears to extend under the 

 marginal plates, and connect with the small plates whicli cover the 

 excavations above. 



Fig. 6 represents the remains of a specimen which was perfect, but 

 imbedded in the rock to such an extent that only eight or nine of the 

 marginal plates and a small portion of the integument of the upper 

 side were visible. It was silicified, but not to such an extent as to 

 resist completely the action of hydrochloric acid. It shewed clearly 

 that the integument was attached to the inner edges of the marginal 

 plates. 



The following description of the figures contains all that can be 

 be said further about these specimens : — 



The specimens, four in number, collected by Sir William E. Logan, 

 shew : — 



1. In fig. 1, an impression of a nearly complete disk, more than an 

 inch broad, with all the marginal ossicles complete. They form very 

 nearly a circle, deficient only at one point, a, which may be only 

 accident, or possibly as both the outer rim and inner series of ossicles 

 are deficient at this point, it may indicate the position of a stem or 

 attachment. At one other point, h, the margin is broken, but the 

 inner row of ossicles appears to be continuous, so that if there be 

 not an attachment at a — and no trace of any appendage exists — the 

 animal was most probably free. 



2. Fig. 2 shews the marginal plates, and in a good state of preser- 

 vation, of more than three-fourths of the rina;. There are twentv-six 

 of these, and their form is rudely sub-quadrate, rounded on the inner 

 margin, truncate and bilobed on the outer ; each is divided into two 

 conspicuous halves, of which the inner is convex, granuloso-tuber- 

 culate, and with close, parallel, vertical striae on their sides (fig. Zh) ; 

 the outer half, on the contrary, is deeply excavate, smooth, divided 

 by a radiating ridge mto two shovel-shaped portions, which at their 

 inner base are each deepened into a circular pit, with a tubercle 

 in it (as seen more plainly in the cast, fig. 6 and a). The outer free 



