80 
PROFESSOR OWEN ON THE MOLAR TEETH, 
crown, the compression being from side to side, or from within outwards, and the exten- 
sion of the crown from before backwards. In this direction the crown expands as it rises 
to an antero-posterior breadth of 30 millims. ( = 1 inch 2 lines), whence it contracts, rising 
to a submedian obtuse apical summit. The outer side of the crown is convex from its 
fore margin to two-thirds of the way back, then becomes concave to a vertical ridge, b , 
marking off a short posterior tract of the crown which inclines inward and is almost flat. 
From this tract the continuation of enamel bends abruptly inward and forward (fig. 2, c), 
rapidly sinking to a mere basal ridge, continued along the inner side of the crown into 
the similarly bent anterior border of the crown, e. The concavity bounded by those 
inwardly inflected borders of the crown is divided into two by the prominence of the 
thickened mid parts ( d ) of the crown forming its apex, a. The abraded surface of this 
tooth forms a sinuous tract of dentine, thickest at the middle, thinnest behind (fig. o,p 2). 
The next tooth (p 3) resembles p 2, with increase of thickness, but none of fore-and-aft 
extent. The facet of the crown behind the outer ridge (fig. 3, b) passes more directly 
inward, so as to form the posterior part of the crown. The inner wall ( c ) is more abruptly 
continued from it, subsiding to the ridge crossing the base of the mid inner convexity 
(d), to which the anterior inflected fold of enamel ( e ) is continued. The convexity ( d ) 
is broader and rather flattened between the better defined hollows of that surface of the 
crown. This tooth is implanted by two fangs. 
The third tooth (p 4) adds increase of fore-and-aft extent to that of thickness of the 
crown, and also resembles the succeeding true molars in being divided into two lobes by 
a vertical indent of the outer surface (fig. 1, p 4) entering the inner mid convexity, 
fig. 3, d. This gives to the grinding-surface (p 4, fig. 3) the form of two consecutive cres- 
cents, of which the hinder one is the largest ; such being the type of lower grinder 
common to Rhinoceros and Palceotherium with this singular South- American Perisso- 
dactyle. 
The first of the true molars (m 1 ) exemplifies its earlier development and longer usage 
by having the crown worn down below the convexity and concavities on the inner surface ; 
a broad bilobed tract of dentine shows the outer mid indent, with a remnant of the 
postinternal cavity (fig. 3, ml): the enamel is now reduced to a very thin line along the 
anterior and inner sides of the crown. 
In the penultimate molar (m 2) the sinuous configuration of the inner surface of the 
crown is preserved, the enamel of the mid convexity rising to form the most prominent 
part of the grinding-surface (figs. 1 & 2, d). The tooth is increased in all dimensions. 
The last molar (m 3), with a fore-and-aft extent of crown of 48 millims. (=1" 10'"), 
preserves the same bilobed type as in Rhinoceros , without any trace of a third posterior 
lobe or ‘ talon,’ as in Palceotherium. The lobes have been worn by mastication to a 
breadth of two or three lines, and as the inner enamel-wall is continued by the inflected 
and subsiding fore and hind borders to the inner basal ridge, the crescents of the 
masticating surface have an enamel border only on their outer or convex sides. 
Thus Macrauchenia differs from Rhinoceros and Palceotherium in the limitation of 
