
392 THE AMERICAN NATURALIST. [VoL. XXXVI. 
the oarlike appendages of Cyathaspis, Tremataspis, and 
Pterichthys, and the difference between these appendages and 
those of Cephalaspis. 
Moreover, the Pteraspidee approach the Pterichydz more 
closely than do the Tremataspidz in the division of the cephalic 
buckler into separate plates, and in its separation into a true 
cephalic or rostral portion bearing the median eyes, and a 
thoracic one to which the oarlike appendages are attached. On 
the other hand, Tolypaspis, which must be placed close to 
Pteraspis and Cyathaspis on account of the minute structure 
of the shield, shows no trace of a subdivision of its dorsal shield 
into separate plates. 
It seems to me, therefore, that we must recognize four 
subdivisions of the Peltacephalata of about equal value, viz., 
the Pteraspida, Tremataspide, Pterichydz, and Cephalaspidz. 
The Cephalaspidz occupy a somewhat isolated position on 
account of the very peculiar shape of the head and the position 
and character of the appendages, although on the other hand, 
as shown by the connecting form Thyestes, a close relationship 
between Cephalaspis and Tremataspis is indicated by the resem- 
blance between their median, lateral, and postorbital openings. 
The Pterichyda and Tremataspide are bound together more 
closely than before, owing to their possession of oarlike append- 
ages, large centrally placed orbits, and to the arrangement of 
the so-called lateral-line organs and oral plates. 
The Syncephalata. — For the great phylum of the animal. 
kingdom formed by the union of the vertebrates and arthro- 
pods I propose the name Syncephalata. The delimitation 
of the Syncephalata can be only roughly determined, especially 
at the lower end of the phylum. The main stalk consists of 
the Arachnida (including the Trilobita, Merostomata), the Pelta- 
cephalata, and the Vertebrata. The point of divergence from 
the main stalk of such groups as the Insecta, Crustacea, and 
the simplified and aberrant forms, like the Ternicata, Amphioxus, 
Balanoglossus, and others, are of minor importance and do not 
concern us here. 
The justification of the term Syncephalata lies in the fact 
_ that in this vast series of segmented animals the concentration 


