On THE STRUCTUBE AND CLASSIFICATION OF THE TrEMATASPIDAE. 
27 
The Tremataspidae resemble the Arthropods in : (a) their general arthropod appear- 
ance ; b) in the minute structure of the shell ; c) in the structure and arrangement of the 
openings for the eyes and olfactory organs ; d) in the character and arrangement of the oral 
plates; e) in the absence of vertebrate jaws and mouth and in the presence of a small cen- 
trally placed oral opening similar to that of an Arachnid ; f) in the presence of numerous 
pairs of jointed appendages. 
The presence of segmented appendages in Tremataspis and related forais is iudicated 
by the following evidence: 1) the presence of a pair of oar-like and jointed appendages in 
Pterichthys , Bothriolepis, Cyathaspis, Pteraspis, Tolypaspis and Tremataspis , unlike those of 
any true vertebrate; 2) the presence of a fringe of jointed and movable appendages (23 — 
30 pairs) along the ventral margin of the trunk of Ceplialaspis. 3) the presence of a pair of 
crushing mandibles like those of an Arthropod in the head of Cephalaspis ; 4) the presence 
in Pterichthys and Bothriolepis and Tremataspis of oral plates that appear to be movable 
laterally, like the jaws of an Arthropod ; 5) the presence in Tremataspis of a sériés of eight 
otlier pairs of openings like the ones to wliich the large swimming appendages are attached ; 
6) the presence in Tremataspis of two pairs of entapophyses wliich, like those in Limulus 
apparently serve for the attachment of muscles moving several pairs of appendages ; 7) the 
presence in Cyathaspis and Pteraspis of froni four to six radiating grooves on the inner sur¬ 
face of the dorsal shield, wliich like similar markings in Limulus indicate the points of at¬ 
tachment of dorso-ventral muscles moving several pairs of appendages. 
The concurrent testimony, from so many différent and independent sources, to the fund¬ 
amental similarity between the Tremataspidae and the Arthropods establishes beyond 
question that the resemblance betw r een them it due, not to any incidental parallelism, or to 
mimicry, but to genetic relationship and comnnmity of origin. It could not hâve been a re 
mote relationship, signifying merely a common origin of both types from some anneliden an- 
cestor, because the resemblance consiste in the common possession of highly specialized 
structures characteristic of the fully established Arthropod and Vertebrate types. 
The genetic relationship, therefore, between the Tremataspidae and the Arthropods 
сап mean mothing less thau the dérivation, through changes in structure and function, of 
one type from the otlier. 
But we must not too hastily conclude that the Tremataspidae 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 significence their structure lias 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 from the 
Cephalaspidae , as is conclusively shown by the similarity in the structure of their orbits 
or from the Pteraspidae and Asterolepidae, as is shown by the appendages and the lateral 
line markings ; or from either, as is shown by the microscopie structure of the shell. The 
whole group must be kept together. But, as it is quite impossible to include in the 
4 * 
