
No. 425.] THE TREMATASPIDA. 391 
by hard convex coverings continuous with the outer layers of 
the shell. The median eye was large, complex, and important 
functionally. It consisted of two pairs of ocelli, one pair com- 
pletely united in the median line and the other nearly so. The 
lateral eyes were reduced in size and in functional importance. 
The nasal pit was unpaired and, in some cases, situated behind 
the median eye. The mouth was small, circular, and situated 
near the center of a group of oral plates. No upper and lower 
jaws were present. 
The head of the Peltacephalata may be regarded as a 
modification of the cephalothorax of an arthropod ancestor, 
consisting of three principal groups of segments, namely: the 
precesophageal, including all the parts derived from the cephalic 
lobes; the true thoracic segments; and the highly modified 
vagus segments, formed by the forward migration and their 
complete union with the thorax of from two to four abdominal 
segments. The brain, like that of vertebrates, probably con- 
sisted of three groups of neuromeres derived from these three 
sources. | 
The Peltacephalata have their nearest relatives among the 
known invertebrates in the Trilobita and Merostomata, hav- 
ing retained to a considerable extent the general shape of the 
body, the structure of the head, and the mode of life character- 
istic of these arthropods. 
Sufficient data are as yet unavailable for a permanent arrange- 
ment of the Peltacephalata into orders and families, but some 
modifications of the old arrangement may be made to advantage. 
The old subdivisions into Osteostraci and Heterostraci, pro- 
posed by Lankester, should be abandoned, as they do not mark 
natural divisions. The discovery of heavily armored oarlike 
appendages in Cyathaspis and Tremataspis, and their probable 
presence in Pteraspis and Tolypaspis, unite these genera more 
closely with one another and with the Pterichyde than ever 
before. It is therefore inadvisable to isolate the pteraspidian 
section merely on the absence of multipolar bone cells, unless 
the Pterichthydz are united with the remaining families 
under the heading Osteostraci. But such an arrangement 
would not sufficiently emphasize the resemblance between 
