486 THE AMERICAN NATURALIST. [VoL, XXXII. 
Basal margin formed by the edges of external and internal 
laminz, of which the edges are broken and irregular. From 
the Hamilton Limestone, Waverly, Iowa.” 
(3) R. excavatus.— Our knowledge of this species is con- 
fined to the single imperfect dental plate described by New- 
berry,! and recognized. by him as belonging to the left ramus 
of the lower jaw. Perfectly preserved specimens are very rare, 
it would seem, as most of the material collected by Messrs. 
Teller, Monroe, and Slocum from the Cement quarries of 
Milwaukee are deficient to a greater or lesser extent. 
As far as can be learned from the materials at hand, only 
the lower dental plates are excavated along the cutting edge in 
the manner described by Newberry, and the sinus varies some- 
what in length among different individuals. The lower jaw is 
further characterized by having an inferior projection at the 
symphysis, as in Ptyctodus. It is greatly prolonged downward, 
being, in fact, spiniform, and recalling the conditions in 2. 
ferox; in one of Mr. Monroe’s specimens it occupies fully 
half of the front margin. Of what practical advantage such a 
contrivance could be it is difficult to perceive. Vermiculating 
furrows do not occur on the surface of well-preserved speci- 
mens, but may be sometimes brought out through corrosion or 
abrasion. The outer surface is normally smooth, or is marked 
only with very fine concentric striz. 
Lower dental plates have the cutting edge beveled off on 
the inner face only. Upper dental plates show distinct traces 
of wear on the outer face, which terminate abruptly, however, 
at a slight distance behind the anterior beak. This proves 
that the beaks in upper and lower jaws were not directly 
opposed to one another, but those of the upper protruded in, 
front of the lower when the mouth was closed. We are led to 
infer from the conditions in R. secans that the beaks in both 
jaws were more or less similar, but as none of the upper beaks 
are completely preserved in the material at hand, this inference 
must remain for the present unconfirmed. The cutting edge 
1 Rep. Geol. Surv. Wisconsin, vol. ii (1877), p. 397- Annals N.Y. Acad. Sci- 
ence, vol. i (1878), p. 192. Mon. U.S. Geol. Suru., vol. xvi (1889), p. 5% Pi. 
CXIX, Fig. 1 
