FISHES OF THE DEVONIAN AGE. 65 
in the defensive armor of D. Terrelli, and I have suspected that, like Tita- 
nichthys, the body was perhaps protected below by a single great shield. I 
have also a plate of D. Hertzeri, which, though incomplete, is triangular in 
outline, nearly three feet long and a foot wide at the broader end. This 
probably constituted the protection of the side of the body, but nothing 
just like it has been found in connection with the species of Dinichthys from 
the Cleveland shale. These suggested rather than demonstrated differences 
of anatomical structure have led me to think that, when all the plates and 
bones of D. Hertzeri shall have been found, they will show divergencies from 
D. Terrelli which will perhaps be thought to have generic value. This 
question, however, cannot be decided at present, and will perhaps remain 
for the consideration and decision of palzeontologists of another generation. 
I have elsewhere referred to the discovery of the dorsomedian plates of a 
large Placoderm in the Huron shale of Kentucky, and also to the discovery 
in the same formation at Louisville of large fragments of bone which have 
not yet been classified It is evident, therefore, that the southern extension 
of the Huron shale offers a field for future exploration from which much is 
to be hoped, particularly for the complete elucidation of the structure of 
Dinichthys and Aspidichthys. 
HETERACANTHUS, nov. gen. 
Pectoral (?) spines eight inches or more in length, robust, with a pos- 
terior opening reaching to or near the summit; base compressed, one and a 
half inches wide, obliquely rounded below, shaft curved forward, regularly 
arched transversely, covered with highly polished enamel, and marked by 
fine denticulate longitudinal sutures, which divide the surface into broad 
nearly equal bands or flattened ridges. The sutures are most numerous 
below, but terminate in succession above, so that few reach the conical 
pointed summit. 
These spines are quite unlike any heretofore found in our Paleozoic 
rocks. They will be recognized at once by their want of symmetry, re- 
versed curve, smooth and polished surface, and sinuous or denticulate longi- 
tudinal sutures. 
MON xvVI——5 
