No. 428.] PALEOZOIC FISHES. 655 
Shale [D. herzeri and D. terrelli]. The age of the beds from 
which this specimen was derived is unquestionably not later 
than the Hamilton, as is shown by associated Brachiopoda, 
etc., characteristic of the Hamilton of Iowa and Illinois. The 
present specimen, therefore, affords the earliest indication of 
this genus yet discovered. 
“The characters displayed by the Illinois specimen, which 
serve to distinguish it from the Ohio forms, may be summarized 
briefly as follows: 
“1, The dental plate under discussion, while resembling 
D. herzeri in general form and proportions, differs from the 
latter in the character of the cutting edge, which is destitute 
of the series of conical denticles occurring in that species. 
The dentary portion of the mandible is similarly well demar- 
cated from the spatulate posterior shaft, and is similarly rounded 
along the front margin. The anterior beak is much produced, 
irregularly lozenge-shaped in transverse section, with a sharply 
rounded anterior angle and a sharp posterior cutting edge. A 
deep notch separates the cusplike beak from a low, round- 
crested prominence in advance of the cutting edge of the 
dentary, which is deeply beveled along its outer face by attri- 
tion against the upper dental plates. To the same cause is prob- 
ably to be ascribed the gently concave curve of the trenchant 
border-itself. The latter terminates abruptly behind, and along 
the posterior slope of the dentary are to be counted the bases 
of five small downwardly directed denticles. The lower portion 
of the posterior slope is smooth and meets the shaft in an 
obtuse angle. In this denticulation of the posterior slope of 
the dentary, and perhaps also in the presence of an elevated 
prominence or denticle behind the anterior beak, are to be 
found the principal differences between the present specimen 
and D. terrelli. 
“2. The form of the shaft is not unlike that of D. herzeri, 
except that it is more contracted immediately behind the den- 
tary portion, and more regularly arched along its lower border 
before passing into the acutely rounded posterior extremity. 
The general outline of the shaft is spatulate, its outer face 
moderately convex, and it is separated from the dentary portion 
