12 
William Patten. 
of the oral plates points to the conclusion tliat it lies in the centre of the oral région, behind 
the anterior median plate. 
This plate clearly occupies a triangulär depressed area between the first tliree or four 
rows of paired plates, and the dépréssion deepens at the narrow posterior end as though it 
led into a small circular opening, like the mouth of a Limulus or a Scorpion. 
Indeed tliere is no trace whatever of any structural feature indicating the presence of 
uppcr and lower mandibles or of an oral opening, like those in a true Vertebrate. On the 
other hand the arrangement of the paired plates, the small central dépréssion and the mé¬ 
dian triangulär plate, so suggestive of the rostral plate of an Araclmid, indicate the pre¬ 
sence of oral structures like those in au Arthropod. 
This view is strengthened by a considération of the sériés of so called gill openings 
that surround the oral plates. 
As is well known, the large compound plate of the fourth row is provided with a sé¬ 
riés of incisions that lielp to inclose a number of so called gill openings, in the manner de- 
scribed by Rohon, and which my own observations amply confirm. The lateral corner of this 
plate is broken off in the only isolated specimen known, and it is mucli obscured in the cast 
which shows this plate in position. But enough is left to show that the most lateral incision 
was much the largest and helped to inclose the third opening of the sériés. The first two 
openings appeared to be inclosed respectively by the lateral plates of the first and second 
rows. We hâve no certain evidence of this however, as the lateral margins of these plates 
in the cast are partly concealed, and the margin of the dorsal shield opposite these plates 
is only shovvn in specimens wliere the plates are absent. The plates in question however are 
so near the latéral margin, that tliere could not be any other plates beyond them, unless 
they were extremely small. The only doubtful points therefore are the position, and the out¬ 
line of the latéral margin of the marginal plates of the first and second rows. It seems to 
me very probable that they will be found with scalloped margins bounding the first three 
openings, as the posterior lateral plate bounds the last six openings but one. 
The Appendages. What is the significance of the nine pairs of ventral openings? If 
we regard Tremataspis as a true Vertebrate, tliere is obviously no other course open than 
to assume, as Rohon and otliers hâve doue, that they are gill openings, for we known of no 
other vertebrate structures with which they could be compared. But we may not complété 
the vicious circle and daim, as some hâve done, that because Tremataspis lias gill slits, it 
is a true Vertebrate. The discovery of three new openings in front of the old ones places 
the whole subject in another light. If we look at Tremataspis from the side, we see that 
the most anterior opening is visible as a semicircular incision in the margin of the dorsal 
shield, and clearly cornes at the same point as the well Imown openings on the margin of Pter- 
aspis, Oyathaspis and Tolypaspis, which hâve been looked on as the openings for the lateral 
eyes. But they cannot be eye openings in Tremataspis because the latéral eyes are already 
