On THE STRUCTURE AND CLASSIFICATION OF THE TrEMATASPIDAE. 
25 
its nerve, and by the origiu of the nerve in the brain. In Limulus , the organ was unquest- 
ionably originally a visual organ serially homologous witli the lateral and median eyes. 
In the adult it lies on the ventral side in front of the chelicerae. It gradually loses the 
histological characters of a visnal organ and finally présents many points of resemblance to 
the olfactory organ of a Vertebrate. The hypostomeal eyes of Trilobites are very prohably 
homologous with the olfactory organs of Limulus. In Branchipus, the same organ lias moved 
from its original position on the ventral surface to a point on the dorsal surface almost as 
far back as the median ocellus. In Apus, the two organs hâve moved still farther back and 
bave united behind the median ocellus to form an uupaired organ iu precisely the same 
location as the post-orbital opening of Tremataspis. 
G. The Oral Plates hâve been workecl out anew from the single fossil and its cast 
that was used by Bohon. My description differs from bis in several very important respects. 
According to my interprétation of this important fossil, there are on each side nine 
oral plates arranged in four rows. The marginal plates are provided with one or more 
rounded incisions lying opposite corresponding incisions of the ventral and dorsal shields. 
The marginal plate of the fourth row is probably a compound plate. There is a row of four 
or five small plates on each side, just behind the anterior margin of the dorsal shield. 
A small triangulär median plate lies in the anterior oral région. Its anterior margin 
seems to be articulated to the hinge-like process on the anterior median margin of the 
dorsal shield. The outer surface of the plate is smooth and provided with a low keel-like 
ridge that gives it the appearance of the rostrum of an Arthropod. Its apex lies consider- 
ably below the surrounding surface in a median dépréssion that probably leads into a small, 
circular oral cavity like that of an Arachnid. There is no trace of a large transverse oral 
opening between the anterior plates and the anterior ventral margin of the dorsal shield. 
H. The Anterior Margin of the Dorsal Shield is deflected sharply downwards to form 
a low wall in front of the oral région. A small keel lies in the median line on the posterior 
surface of this wall with its rounded edge directed backwards. Two rounded, tooth-like pro¬ 
jections of the rim directed ventrally, lie on either side. The posterior lateral margins of 
the dorsal shield are folded sharply toward the ventral median line and present three large 
rounded incisions that continue forward the sériés of six incisions of the ventral shield de- 
scribed by Schmidt and Rohon. 
I. The Appendages: The most anterior incision is the largest and is clearly the same 
as that so well seen on the margin of the dorsal shield in Tolypaspis, Oyathaspis and Pter- 
aspis , and which bas been regarded as an opening for the lateral eye. Lindstrom ’s im¬ 
portant discovery of an appendage in Gyathaspis , my own discovery of the appendages in 
Tremataspis , and a renewed examination of the Pteraspids in the British Museum iudicate 
Зап. Фаз.-Мат. Отд. 4 
