', age : General Notes. [ March, 
“interstitial” type of tooth of Ryder. They constitute in part the 
intermediate (median denticules) cusps of Gaudry.? The object of 
the cuspule is to support the cusp, to the base of which it is attached. 
Indeed, it presents the first attempt to modify the bunodont type of 
dentition toward the lophodont type. It is consistently main- 
tained only when the strain of impact is of moderate amount. 
Should the strain become excessive the lateral border of the tooth 
: is arched inward, as first pointed out by Ryder.’ In the presence 
= of these strong arches or flutings the main surfaces of attrition 
no longer need the basal cusp supports and they disappear. This 
change is illustrated in Paleosyops and Limnohyus. 
en a tooth becomes worn and an island of dentine appears 
at the summit of the cuspule it is often seen to be obliquely 
-~ placed to the main cusp. This is occasionally seen in Acheno- 
don and Mastodon and constantly in Hippopotamus. A key isin 
this way afforded to interpret the islands of the complex tooth of 
Phacocherus. Care should be taken not to be misled in applying 
this method of interpretation to intricate types of tooth-structure, 
such as Polymastodon and its allies, for in these genera there €x- 
ist true cusps only (cuspules being entirely absent), and the third 
row of islands (when such exists) being simply a linear row of 
well-defined characters placed along the border of the tooth. 
~ A tooth that has been for a long time subjected to the action of a 
dilute acid is entirely deprived of its enamel, and permits the super- 
_ ficies of the dentine to be clearly seen. It is of interest to note 
that the cusps upon the dentine of a tooth thus prepared, while 
corresponding in a general way to those on the free enamel surface, 
E are sufficiently distinct therefrom to afford material for comparisons, 
i and to suggest relationships of a different character from those 
determined by the study of the entire tooth. Thus the dentine 
cusps of the premolars of Sus suggest the form of the correspond- 
ing teeth in Monachus and of Mesonyx. The cusps of the third 
molar of Sus are strikingly like the molars of the Insectivora and 
of lower molars of the genus Bathyopsis. In like manner the 
: i 
t 
$3 essential features of teeth so remote as to recall the type seen in 
‘  Centetes, Gymnura,* Chrysochloris, as well as in the numerous 
ancestral types described by Cope from the North American 
Eocene? | 
_ A practical method of studying teeth with the object in view of 
: : ‘Pris of Acad. of Natural Sciences, 1878, 45. 
z Les Enchainements du Monde Animal, Paris, 1878, 70. 
Les Cit - j 
: * The arrangement seen in Sus is the same essentially as in Gymnura. The minute 
_ cuspule on the crown of the first upper molar is in precisely the same position as 10 
cept that it is not connected to the anterior inner cusp as, named by that author. Itis 
-a little curious that the dental formula of Gymnura is the same as in Sus. nee 
ee 5 Report U. S. Geo. Sur. of Territory, m1, 1884. 
nal surface of an aberrant molar tooth of man presents the . 
Sts eE GETAN E a ieee ea Sed E E tacks igor: ag ET cee 
