110 PALEOZOIC FISHES OF NORTH AMERICA. 
spine is flattened, and when extended its edge is turned forward so as not 
to impede the passage of the fish through the water. As the fin is folded 
it turns, presenting its flat side and the web of the fin to the water, to facili- 
tate propulsion. In Bothriolepis the pectoral spine is also flat and must 
have revolved in the same way; the articulation was also very firm, and 
the spine is usually found in connection with the plates or bones which 
formed the socket. ; 
The peculiar laminated structure of the articulating head of the pectoral 
spine in Pterichthys and Bothriolepis—figured though not mentioned by 
Agassiz and Pander—seems to have been precisely the same as in Plecos- 
tomus, Phractocephalus, and other plated Siluroids, and is visible in Pimelodus. 
This strengthens the suggestion of Professor Huxley, that the Placoderms 
are genetically related to the plated Siluroids of our present day. The 
points of similarity which led him to suspect a relationship between them 
were mainly in the defensive armor and the structure of the shoulder girdle. 
To these I now add the striking analogy in mechanical arrangement and 
identity of microscopical structure in the pectoral spine and its articulation. 
It seems impossible that all these coincidences could have been produced 
by anything but inheritance. 
Prof. EK. D. Cope, in the American Naturalist for March, 1885, proposes 
a new theory of the zoological relations of Pterichthys, namely, that it was 
a/Tunicate allied to Chelyosoma. With the abundant proofs of the relation- 
ship of Pterichthys to Bothriolepis, Aspidichthys, Holonema, and the other 
members of the family Pterichthide, it is evident they must be grouped 
together, and the ichthyic character of Pterichthys is settled by the preser- 
vation in many instances of a tail covered with seales connected with the 
carapace. It is true that in the Canadian species of Pterichthys (P. Cana- 
densis, Whiteaves), of which so many individuals have been found in excel- 
lent preservation, no evidence of a caudal extension of the body has been 
seen; probably for the reason that it was not scaled, but simply covered 
with a skin, like Polyodon among the generally plated Chondrostei. 
Two species of Bothriolepis seem to have left their remains in the Che- 
mung and Catskill rocks of New York and Pennsylvania; one, that just 
referred to, from the Catskill, and another with smaller and more finely 
