On THE STRUCTURE AND CLASSIFICATION OF THE TrEMATASPIDAE. 
7 
median line is a rounded dépréssion of the rim whicli bears on its posterior surface a remark - 
able keel-like ridge. The keel stands vertically, with its tkickened ventral end just protrud- 
ing above the level of the oral plates. Its pointed dorsal end lies close to the inner surface 
of the dorsal shield. When seen from the side, or in sagittal sections, the crest of the keel 
is nearly circular in outline, with the convex surface directed backwards. When the lime- 
stone matrix is deeply excavated in this région, the keel is seen to lie on an indistinct plate, 
which in turn seems to fit into the angle formed by the under surface of the dorsal shield 
and the posterior surface of flexed margin. The plate is somewhat irregulär, but is dis- 
tinctly triangulär in outline, with the apex directed upwards and backwards. 
The surface of the keel, and of the keel-plate, is similar in color and general appear- 
ance to the outer surface of the shield, but differs from it in being rougher and füll of pores 
which make it difficult to remove the matrix. 
On either side of the two teeth-like processes, the free margin of the rims is gently 
undulating and forms a second lobe, c, and a third one e, near the lateral end of the large 
anterior oral plates. 
The free edge of the rim, as far back as the first incision fer the attachment of append- 
ages, présents a perfectly smooth and polished edge without thickenings or change of color 
or texture, showing that these sliglit undulations cannot be compared with tliose more 
strongly marked serrations farther back. 
The Serrations of the Anterior Margin of the Ventral Shield: Rohon lias already 
described accurately, and in considérable detail, the six openings between the anterior mar¬ 
gin of the ventral shield and the oral plates. According to my own observations the anterior 
margin of the ventral shield is bent inwards to form a narrow, shelving wall wliose auterior 
face is thrown into a sériés of flutings. This wall is usually dark in color, and présents a 
characteristic texture, being rougher and more coarsely pitted tlian the polished outer sur¬ 
face of the shield. Yiewed from the outside, the flutings are clearly indicated by the mar¬ 
ginal teetli, separated by semicircular excavations. Where the shield dips down into the 
flutings, there is a slight darkening and thickening of the shell, which often indicates the 
location of the corresponding fluting, even when it is partly concealed. It is certain 
from the specimens in which the oral plates were preserved that the fluted margin of the 
ventral shield indicates the presence of a sériés of tubes or openings, leading into the interior 
of the animal and that the anterior walls of the tubes were formed by the edges of the oral 
plates. The presence of the openings may be indicated either by the fluting of the flanged 
edge of the ventral shield, its coarser porous texture and darker color, or by the narrow 
thickening on the posterior or lateral margin of the opening. These characters hâve been 
used in detecting the presence of openings in front of the six pairs described by Rohon. In 
the several specimens at my disposai I hâve cleaned with great care the anterior lateral 
margin of the ventral shield, and hâve found clear indications of three additional openings. 
