FISHES OF THE CARBONIFEROUS SYSTEM 167 
surface of the plates is, however, so peculiar and strongly marked, that it 
will serve for the identification of even a fragment wherever found. The 
affinities of the genus are apparently closest to Aspidichthys of the Huron 
shale, but more material of both is needed for a satisfactory comparison. 
TRACHOSTEUS CLARKII, n. sp. 
Plate XLII, Figs. 1-8. 
The characteristics of this species are for the most part given in the 
generic description, and its more minute and specific features can only be 
detailed when other individuals shall have been found. The dimensions of 
that which has served as a basis for the generic description can be best in- 
ferred from the under jaws, of which the dentary bones were apparently 
about twelve inches in length. Unfortunately the anterior extremities are 
somewhat broken, and therefore the exact length and the character of the 
symphysis cannot be determined. The posterior portion of each is straight, 
about an inch in width, blunt-pointed or rounded and flattened at the end; 
the anterior portion is nearly smooth without and within, about an inch in 
width, and carries on its upper margin acute, rather slender, teeth, which 
are three-eighths of an inch in length, and are composed, as in all other 
members of the family, of indurated enamel-like jaw-tissue. The premaxil- 
laries are about an inch and a half in length, the anterior portion arched, 
excavated, and pointed, as in Dinichthys. The eye-orbit is one and three- 
quarters inches in diameter and nearly round. The inequality in the breadth 
of the sclerotic plates and the tuberculation of some part of the external sur- 
face will serve at once to distinguish the eye of this fish from that of Di- 
nichthys Gouldii, which occurs in the same beds. The external diameter of 
the ring of sclerotic bones is about the same in both, viz, four inches, but 
in Dinichthys the orbit is elliptical and all the orbital plates are of about 
equal breadth. 
The outlines of the dorsal plate cannot be fully made out from the 
single specimen yet known, but it was apparently oblong, about fifteen inches 
in length by ten inches broad; the anterior (?) lateral plates are unsym- 
metrically ovoid in outline, about six inches long by five broad. 
