MARTIN: A COMPARISON OF THREE SKULLS. 393 
The diastema between the molar series and the massive in- 
cisor tooth differs much comparatively in the Lenawee and 
Boicourt specimens. In the former it measures 110 mm., in 
the latter 140 mm., and is, proportionately to the size of the 
skull, much longer. 
A comparison of the large incisors of the two specimens re- 
veals quite a difference in the markings of the outer enamel 
covering. In the Boicourt specimen the flutings are very 
narrow, and, counted from the anterior inner angle round to 
the posterior sinus, there are twenty-four. Correspondingly, 
the Lenawee skull shows only eighteen, while two of the fluted 
groovings on the anterior face of the teeth are exceedingly 
wide, the widest being 8 mm. from ridge to ridge, and widens 
out materially as it nears the extremity of the tooth. 
In Castor the cutting edge of the incisors is worn nearly 
straight across, with slightly more wear on the outer edges, 
but in the Castoroides quite the reverse occurs, the teeth being 
worn thin and sharp at their junction on the medial line, and 
gradually get thicker and more heavy laterally, the two teeth 
thus forming a very efficient gouge. 
A lateral view of Castoroides, when compared with Castor, 
shows quite a difference in contour of the dorsal part of the 
skull. In Castor the crest from supraoccipital to nasals shows 
quite a convexity, the highest part of which occurs at the 
junction of the parietals and frontals on the median line, and 
more strongly convexed at the caudal end of the nasals than 
at any other point, with the parietal ridge scarcely notice- 
able. In Castoroides the parietal and saggital ridge is very 
conspicuous, and is elevated 10 mm. above the parietals. From 
the supraoccipital ridge to the frontonasal suture on the 
median line it is nearly level; forward from here to the tips of 
the nasals it has a gentle slope. The malar is wider, thicker 
and more rounded than in Castor, with the lower portion 
gently converging inward. The cephalic edge, from the 
superior malomaxillary suture downwards, has a slightly 
more acute angle than has Castor. In Castor the malar on its 
outer face is nearly flat its whole depth from top to bottom. 
The squamosal is relatively much larger than in Castor, and 
extends higher up the sides of the cranium, thereby decreas- 
ing the width of the parietals. The infraorbital foramen in 
