a 
1871.] 85 [Cope. 
is twisted, so that the vertical plate of the triturating surface is quite 
oblique to that of the basal portions of the shaft. 
The triturating surface is, in its long diameter, transverse to the mar- 
gins of the tooth adjacent; the short diameter is very oblique. The 
bulge is well marked, and in the specimens a little anterior to the middle. 
The inner layer of dentine is thickest anteriorly, where it is but a little 
narrower than the thick external layer, but it is nowhere very thin. 
The outer face is concave, a feature not seen in the three species above 
mentioned, and not exhibited by any of the sections of the teeth of MW 
jeffersonti given by Leidy, 1. c. 
M. 
Length of fragment of tooth........ ES CO ae Vices aso ouuenss 0.043 
Long diameter grinding surface.......... ee es es <e wUROe 
uoty xv A ae Levee gosta 
These dimensions show that the Megalonyz tortulus is not larger than 
M. sphenodon, perhaps not so large, as the diameters of the apices of 
their teeth are identical, while that of the base is equal to the apex in 
the former, greater in the latter. The concavity of the outer face, and 
disposition of the dentine, are entirely different from that seen in J. 
and sphenodon other species, and more as in ©. jeffersonti and M. loxodon. 
For the better discrimination of these species, the following synoptic 
table of dental characters is added. 
A Canine-molars, much curved, of equal diameter. 
Large, bulge median ; grinding surface oblique. M. jeffersontt. 
Large, bulge anterior; grinding surface a groove. M. loxodon. 
Small, concave externally. M. tortulus. 
zp Canine-molars little curved, of uniform diameter. 
Molars triangular, canine-molars less compressed, large. J. wheatleyt. 
Last molar oval, canine-molars more compressed, large. MM. dissimilis. 
c Canine-molars little curved, diameter contracting to the apex. 
Bulge median, dentine thin within, small. M. sphenodon. 
Mytopon, Owen. 
MyLODON ? HARLANI, Owen. 
The remains representing this genus are not sufficiently characteristic 
to enable me to determine the species with certainty. They consist of two 
imperfect ungueal phalanges, and the distal extremity of the tibia. The 
former indicate a very large animal; they are stout, convex above, with 
lateral ridge and three basal plates. The flexor insertion is broad and 
flat, the foramina well developed. Inthe second phalange the middle 
inferior plane is represented by an obtuse angle. The tibia presents the 
excavation for the astragalus, as in J. robustus Ow,* but is narrower 
or with less anteroposterior diameter than in that species. 
*See Owen on Mylodon. Vl. xx fig. 4. 
