On THE STRUCTURE AND CLASSIFICATION OF ТПЕ TrEMATASPIDAE. 
11 
identical with R о lion’s. These plates are more convex and robust than tlie one we hâve just 
described and more than twice too large to fit into its place. Moreover, there is no corre- 
spondence in shape or texture between their anterior margins and the edge of the dorsal 
shield with which they must hâve articulated, if tliey occupied the position assigned to them 
by Rohon. 
In the most perfect specimens, the anterior margin is thickened, of a different color, 
and with a dull roughened surface, as thougli it were attached by means of a tough membrane, 
or by sinews, to somë fixed plate. 
A small scale in the University Collection at Petersburg evidently belonging to Tre- 
mataspis , is interesting in this connection (PI. I. fig. 7). It is a triangulär plate with its 
inner concave surface exposed (8 mm. long and 4 mm. wide). It at first appeared to be a 
median plate of symmetrical form, and possibly fltting into the oral région. A second and 
better plate, belonging to the Dartmouth collection, lias been exposed during the prépara¬ 
tion of this manuscript. A careful examination indicates tliat it is not bilaterally symmet¬ 
rical and cannot represent an isolated rostral plate. And indeed it is much too large to fit 
into the place occupied by that plate. 
It is therefore out of the question to place either of these triangulär plates in the me¬ 
dian oral région, since neither plate lias the size, shape or articulating surfaces necessary to 
fit into such a place. Moreover if the convex triangulär plate belongs in the place assigned 
to it by Rohon, it must hâve been articulated to the anterior rim of the dorsal shield, and 
thus completely closed the place wliere Rohon supposed the mouth was situated. 
It is clear, therefore, that neither of the two triangulär plates that hâve been found 
separately can be identified with the median oral plate we hâve found in situ. If they belong 
to Tremataspis , and there is little room to doubt that, they must hâve belonged to so me 
other part of the body, most-likely to a pair cf movable appendages like those of Pterich- 
thys. These plates will he described more fully in a subséquent publication. 
There is no trace whatever of the large semilunar space figured by Rohon ’93 in PI. I. 
fig. 8, and which he regards as the mouth. If he had worked out more carefully the folded 
anterior margin of the dorsal shield, he would hâve liardly feit justified in placing the mouth 
in this position. It is true that in the cast we are describing, the anterior margin of the 
ventral shield is not certainly visible, but in front of the anterior row of oral plates is a row of 
four or five small quadrangular plates that Rohon failed to figure or describe. These plates 
are either the crushed margin of the dorsal shield, or else a row of independent plates, their 
regularity in form favoring the latter view. But in any case no space is left here for such 
an oral opening as Rohon describes. Moreover there is an insuperable objection to placing 
the mouth in a position where it must always be closed. For the upper jaw being a part of 
the dorsal shield must be stationary and the supposed lower jaw could not be moved without 
dislocating the remaining oral plates. 
Where tlien is the mouth of Tremataspis? It seems to me that the whole arrangement 
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