386 THE AMERICAN NATURALIST. | [Vor. XXXVI. 
five small plates on each side, just behind the anterior margin 
of the dorsal shield. 
A small triangular median plate lies in the anterior oral 
region. Its anterior margin seems to be articulated to the 
hinge-like process on the anterior median margin of the dorsal 
shield. The outer surface of the plate is smooth and provided 
with a low, keel-like ridge that gives it the appearance of the 
rostrum of an arthropod. Its apex lies considerably below the 
surrounding surface in a median depression that probably leads 
into a small circular oral cavity like that of an arachnid. There 
is no trace of a large transverse oral opening, like that described 
by Rohon, between the anterior plates and the anterior ventral 
margin of the dorsal shield. 
The anterior margin of the dorsal shield is deflected sharply 
downward to form a low wall in front of the oral region. A 
small keel lies in the middle line on the posterior surface of the 
wall, with its rounded edge directed backwards. Two rounded 
toothlike projections of the rim, directed ventrally, lie on either 
side. The lateral margins of the dorsal shield are folded sharply 
toward the ventral median line and present three large rounded 
incisions that must be regarded as a forward extension of the 
series of six incisions of the ventral shield described by Schmidt 
and Rohon. 
The Appendages. — The most anterior incision is the largest 
and is clearly the same as that so well seen on the margin of 
the dorsal shield in Tolypaspis, Cyathaspis, and Pteraspis, and 
which has been regarded as an opening for the lateral eye. 
Lindstróm's important discovery of an appendage in Cyathas- 
pis, my own discovery of fragments of the appendages in Trema- 
taspis, and a renewed examination of the pteraspids in the 
British Museum indicate that in these four genera the large 
anterior marginal incision served for the attachment of an oar- 
like appendage similar to that in Pterichthys and Bothriolepis. 
The remaining openings, which are unquestionably serially 
homologous with the first, must have served for the attachment 
of other appendages of a similar nature. They decreased in 
size from before backwards, and were pn too delicate to be 
well poonvel) in a fossil condition, 

