8 
William Patten. 
The first specimen carefully studied, PI. I fig. 1 shows three somewhat broken and distortcd open- 
ings g , /г, ij, on the left side aud in front of the sériés described by Rohon. The figure, although 
carefully drawn, liardly does justice to a préparation so difficult to reproduce faithfully and 
accurately. Two otlier préparations at Dartmouth show substantially the same condition. 
We must therefore now recognize, certainly eight, probably nine pairs ofopenings, arranged 
in nearly a semi-circle around the oral région. The great difficulty in deterinining the exact 
number is in obtaining a préparation showing the whole sériés in place. It is possible, but 
I do not regard it as probable, that the rnost posterior of the three new incisions coincides 
with the most anterior one described by Rohon. The incisions gradually increase in size 
from behind forwards, the first five being on the anterior margin of the ventral shield, the. 
next three on the median side of a narrow strip that may be regarded as a continuation for¬ 
wards of the ventral shield; wliile the most anterior and largest opening is eut out of the 
lateral margin of the dorsal shield. The position of this last opening is clearly shown in the 
side view of the model. 
The Oral Plates. That the large ventral opening in the liead of Trematasjois is in part 
filled by a number of remarkable plates was first shown by Rohon in 1892, in his description 
of a remarkable specimen preserved in the Petersburg Academy, the only specimen in whicli 
such plates hâve been found in place. 
When it was my privilège to examine this specimen for the first time, I had some 
doubts whether it had the value and importance attributed to it. Further study, however, 
convinced me that its value had been underestimated, rather tlian otherwise, and I began to 
inake as careful and painstaking a study of ail its details as lay in my power. I am conse- 
quently now able to add new and important data to our knowledge concerning it and to put 
upon it an interprétation that differs in sonie essential respects from that of Rohon. 
Unfortunately, in this specimen, the anterior margins of the dorsal shield were either 
destroyed or absent, so that the relation of the plates to them and to the three anterior pairs 
of ventral openings could not be determined. 
In constructing the model from which the figures were made the boundaries of the 
oral région were first reproduced from several different specimens showing tliese parts. Then 
from the original cast of the fossil, several impressions were taken in dentist’s wax, which 
gave a faithful reproduction of the original fossil. The impressions were then modelled in 
bee’s wax on an enlarged scale and finally copied on the open area of the model of the 
whole liead. It then required only trifling alterations of some lines to make the whole sériés 
of plates fit into the area that had been left open for them. This was true, however, of the 
left side only. The plates on the right side were considerably crushed and in part absent, 
so they were simply made to duplicate those of the opposite side, but without the details due 
to distortion or the fracture of the plates. In otlier words, the oral plates of the left side are 
copies of those in the original fossil, wliile the right side is largely a duplication of the left. 
