Dr. R. H. Traquair — On Drepniiaspis. 157 



specimens, whence it will be seen that it presents a narrow median 

 notch, the direction of which is continued for a little way in front 

 by a longitudinal fold-like elevation of the surface. 



Returning now to the front, we find on each margin of the 

 carapace, immediately behind the small external labial plate of the 

 mouth, another and rather larger element of a trapezoidal shape 

 (a;.), transversel}' placed, with a short external margin, a longer 

 internal one, while the posterior is the longest and is directed 

 nearly at a right angle to the middle line of the creature. In the 

 outer extremit}' of this plate is a small rounded depression, which 

 in the specimen sketched in Fig. 2 and in some others is seen on 

 the ventral aspect of the fossil ; but as it also appears in some on the 

 dorsal aspect it appears to me to have been marginal, and that 

 the outer extremity of the plate on which it occurs must have been 

 reflected upwards Avhen in an uncrushed condition. In my 

 " Silurian " Memoir above quoted, I have designated this depression 

 as an orbit with a (juery, and it certainly occupies a position 

 singularly analogous to that of the supposed orbit in Fleraspis, 

 which is also very small in proportion to the size of the animal. 

 But in some examples before me it seems to have a floor furnished 

 with tubercles similar to those of the rest of the surface, so that its 

 orbital nature can hardly be maintained. However, from its constant 

 occurrence on the same plate and in the same position it must have 

 a meaning, and doubtless it marks the place of an organ of sense of 

 some sort, which can hardly be a nostril, and as the plate itself 

 must have a name we may call it the sensorij plate. 



Immediately behind this ' sensory ' plate is another and larger 

 one (a.v.l.) of an approximately triangular form, a long irregularly 

 scolloped side internally, a gently convex outer one, and an acutely 

 pointed posterior angle. The outer margin of this plate, which we 

 may call anterior vetitro-lateral, fits on below the anterior pointed 

 extremity of the great postero-lateral plate (p-l.), so extensively 

 seen on the dorsal surface (Fig. 1), but wliich appears on the 

 ventral aspect simply as a rather narrow thickened margin, marked 

 with wavy ridges instead of tubercles, and is obliquely bevelled off 

 just at the postero-lateral angle (Figs. 2 and 3). These lateral 

 elements on the ventral surface are on each side separated from the 

 mental and median ventral plates by a series of small polygonal 

 ones, as represented in Figs. 2 and 3, but just behind there is an 

 ovate-oblong one of a considerable size (p.v.L), which may be called 

 posterior ventro-lateral. The space between this and the postero- 

 external angle of the plate p.l. (left empty in the figures) seems 

 in one specimen to be covered by another smaller one ; any way, 

 I think that in this x-egion the branchial aperture must have been 

 placed, though its position is as yet not exactly determined. 



In a specimen in the collection of the Geologische Landesanstalt in 

 Berlin, the posterior notch of the median ventral plate seems to 

 form, along with an elevated median scale just behind it, a narrow 

 opening, which I take to be the orifice of the cloaca (see Fig. 3). 

 This is succeeded in the backward direction by four similar scales- 



