10 
William Patten. 
The second row of plates is mucli more clearly sliovvn. It consists of at least three 
distinct plates: a small triangulär inner one, a larger median pentagonal one, with its long 
axis in a transverse direction, and an irregulär marginal one witli its long axis nearly parallel 
with the margin of the dorsal sliield. The last two plates and tlie marginal one of the 
first row hâve been forced outward so tliat their adjacent ends form a pyramidal 
élévation. 
The two plates in the third and in the fourth rows are very clearly shown. The large 
size of the posterior marginal one and the presence of a well marked suture on its anterior 
border indicate that it is a compound plate. The suture extends nearly half way across and 
then bifurcates, as though three plates were nearly fused in one. If tliis is really the casé, 
tlien we must assume that the third and fourth rows consisted originally of three plates eacli, 
arrauged as indicated in the figure. 
Ail the oral plates were apparently united by a tough but flexible membrane that 
would allow some freedom of movement. This is indicated by the polished and rounded 
edges of the plates and by the rounded contours of the matrix in the intervals betvveen thein. 
On the right side of the fossil, the median plates of the third and fourth rows are in a 
nearly normal position, and they are so well preserved that they fix beyond any doubt the 
exact location of the median plane. The first and second rows on the right side are so badly 
crushed and depressed that their arrangement cannot be made out. One can distinguish, 
however, a longitudinal row of four or five overlapping plates, or fragments, that perhaps 
correspond to the inner plates of the second and third rows on the opposite side of the 
median line. 
The Median Plate is such an extraordinary structure that it deserves careful considér¬ 
ation. It occupies an elongated triangulär area between the first three rows of plates. It 
lies considerably below the level of the left side, which has evidently been thrust ventrally 
and toward the median line, thus making a conspicuous longitudinal fold which partly 
overlaps the right side of the plate. The latter is triangulär in outline with a smooth outer 
surface like that of the otlier plates. Its posterior pointed end is slightly rounded; the 
bounderies of its broad anterior end cannot be distinguished. A shallow groove runs along 
the right margin while a rather prominent ridge or keel runs along its médian line. The 
ridge is highest at about the middle point of the plate and on either side of this point is a 
slight bulge in its lateral walls. A faint line, or groove, appears to eut off the right anterior 
corner. Although these details are minute, I am confident they constitute normal structural 
features of this plate. 
Unfortunately the relations of the median plate to the anterior margin of the dorsal 
shield could not be determined with certainty. There is, however, little doubt that 
it is attached to the median keel-like process on the under side of the dorsal shield. 
Rohon has figured and described a triangulär plate found separately, which he sup- 
posed might fit into this région (PI. I., figs. 12 & 13). I hâve in my own collection a plate 
