166 PALEOZOIC FISHES OF NORTH AMERICA. 
of wear. The surface of attachment to the cranium of these dental plates 
is flat or concave and somewhat rough, from the coarse cellular tissue of the 
bone; the sides are straight or beveled, apparently for co-adaptation, and 
by this character favor the conclusion that the dentition consisted of many 
pairs of plates, constituting a tessellated pavement; the crowns of the teeth 
below being convex, those above concave. 
Formation and locality: Cleveland shale; Sheffield, Ohio. Collected by 
Mr. Jay Terrell. 
TRACHOSTEUS, nov. gen. 
Placoderm fishes of medium size, belonging to the family of the Di- 
nichthide. Body inclosed in defensive armor, consisting of a number of 
large, but relatively thin, bony plates, of which the outer enameled surface 
is thickly set with high conical tubercles, that are acute, rounded, or cupped 
at the summit. The spaces between these tubercles are radiately lined. 
The form of the head is not distinctly shown in the only specimen yet found; 
the plates of the body consist apparently of one large oblong dorsomedian 
with several smaller and irregular lateral plates united with each other and 
the dorsal by overlap joints. The under jaws, as in all of the Dinichthide, 
consisted of cartilaginous angular and articular parts with dense bony dent- 
ary portions. The dentary bones are nearly straight; the posterior end is 
spatulate and was evidently once covered; the anterior third or exposed 
portion carries a row of slender, conical, acute teeth along its upper margin; 
premaxillaries subtriangular in outline, the anterior face arched, and ter- 
minating below in an acute point; the posterior edge horizontal, and carry- 
ing slender, pointed teeth, which matched with a portion of those of the 
mandible. The eye-orbits are relatively large and round, and are encircled 
by a ring composed of four sclerotic plates, of which those of one side are 
much narrower than the others. The exterior surface of these plates is in 
part tuberculated like the cranial and dorsal plates, in part smooth or radi- 
ately striated. ; 
Only one specimen of the fish to which the above name is here given 
has yet been found, and of this the parts, though all present, are so confused, 
that it is not possible to describe them fully. The ornamentation of the 
