392 KANSAS UNIVERSITY SCIENCE BULLETIN. 
the high pampas of South America. In tooth structure it more 
closely resembles this form than any other of the rodent family 
I have been able to examine, while the long diastema between 
the incisors and the molars, and rapid divergence of the molar 
series posteriorly and the general form of the basioccipital 
region also appear more Bizacacha-like. Although the molar 
teeth of these two forms look very similar on their grinding 
surface, there is quite a difference in their conformation. Those 
of the Bizacacha are made up of two lamine of dentine and 
enamel in molars 1, 2, and 3, the fourth and last having three 
layers. The enamel does not form a true cylinder around the 
dentine, but only reaches part way around, there being no 
enamel wall on the posterior portion of the segments. In 
Castoroides each tooth has one more layer, a tooth being made 
up of tube-like sections composed of enamel and dentine, 
pressed nearly flat and fastened together with a layer of ce- 
ment. 
The dorsal surface of the skull is almost flat, broadening out 
posteriorly, and in the region of the lamboidal ridge is rela- 
tively more broad and massive than in Castor. 
The narrowest part of the skull occurs just behind the 
cephalic ends of the parietals, instead of across the frontals 
as in Castor. Across the frontonasal sutures it presents a more 
massive appearance, and from here it tapers slightly to the 
tip of the nasals, which end rather abruptly and rounding. 
The facial portion of the maxilla composes a larger part of the 
zygomatic arch than does that element in Castor, and forms 
the major part of the front wall of the orbit. In consequence 
of this, the malar does not reach nearly so far forward, and 
has no contact with the lachrymal, while the contrary exists 
in Castor, where the front portion of the malar forms half of 
the anterior wall of the orbit. The squamosal extends further 
forward and commences higher up on the cranial portion than 
it does in Castor, and occupies about two-fifths the entire 
length of the skull, while the nasals are relatively more broad 
than long. The long and narrow parietal ridge rises sharply 
from the flat parietals to a height of ten millimeters. The in- 
terparietal is a relatively small element, almost unobservable, 
fitted wedge-like between the posterior ends of the parietals. 
The parietals themselves are long and very narrow, scarcely 
exceeding in width that of a full-grown beaver. The infra- 
orbital foramen is located higher up the face than in Castor. 
