On the structure and classification of the Tremataspidab. 7 



median liue is a rounded depression of the rim which bears on its posterior surface a remark- 

 able keel-like ridge. The keel stands vertically, with its thickened 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 region, 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 irregular, but is dis- 

 tinctly triangular 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 full 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, presents a perfectly smooth and polished edge without thickenings or change of color 

 or texture, showing that these slight undulations cannot be compared with those more 

 strongly marked serrations farther back. 



The Serrations of the Anterior Margin of the Ventral Shield: Rohon has already 

 described accurately, and in considerable 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 whose anterior 

 face is thrown into a series of flutings. This wall is usually dark in color, and presents a 

 characteristic texture, being rougher and more coarsely pitted than the polished outer sur- 

 face of the shield. Viewed from the outside, the flutings are clearly indicated by the mar- 

 ginal teeth, 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 series 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 have been 

 used in detecting the presence of openings in front of the six pairs described by Rohon. In 

 the several specimens at my disposal I have cleaned with great care the anterior lateral 

 margin of the ventral shield, and have found clear indications of three additional openings. 



