
No. 425.] THE TREMATASPIDE. 389 
parallelism, nor to mimicry, but to genetic relationship and 
community of origin. It could not have been a remote relation- 
ship, signifying merely a common origin of both types from some 
annelid ancestor, because the resemblance consists in the com- 
mon possession of highly specialized structures characteristic 
of the fully established arthropod and vertebrate types. 
The genetic relationship, therefore, between the Trematas- 
pida and the arthropods can mean nothing less than the 
derivation, through changes in structure and function, of one 
type from the other. 
But we must not too hastily conclude that the Tremataspidz 
are true arthropods, and by transferring them from one group 
to the other avoid the real problem at issue. We cannot so 
easily ignore the profound significance their structure has for 
the origin of vertebrates. Their vertebrate affinities are too 
obvious to be denied. 
It is quite out of the question to separate the Tremataspidae 
very far, either from the Cephalaspide on the one hand, as is 
conclusively shown by the similarity in the structure of their 
orbits, or from the Pteraspide and Pterichydz on the other, 
as is shown by the appendages and the lateral-line markings, 
or from either, as is shown by the microscopic structure of the 
shell. The whole group must be kept together. But, as it is 
quite impossible to include in the arthropods a group of animals, 
nearly all of which have osseous dermal skeletons, and some 
of which have such characteristically vertebrate bodies and fins 
as the Cephalaspide and Pterichydz, or to include in the ver- 
tebrates animals having many pairs of jointed appendages, it 
becomes necessary to create for them a new class, one that 
shall occupy a position between the true vertebrates and arth- 
ropods, and unite these two great groups into one compact 
phylum. 2 
I propose for this new class the name Peltacephalata, and 
include in it forms like Pterichthys, Cephalaspis, Pteraspis, 
Tremataspis, and related genera. The class may be provi- 
sionally characterized as follows: 
The Peltacephalata were arthropod-like animals, moving 
about through the soft mud on the bottom of shallow waters, 
