PROFESSOR OWEN ON SOME SPECIES OF THE EXTINCT GENUS NESODON. 303 
and more prominent than the larger and less convex posterior lobe ; it is slightly 
indented on the inner side. The posterior lobe shows the two islands of enamel 
a and h, fig. 14, Plate XVII. on its grinding surface and the terminal fossa (c) of the 
internal longitudinal fold almost reduced to the state of an island. The length of 
the crown is 9 lines, the breadth (now the antero-posterior extent) is 1 1 lines, the 
thickness of the worn surface of the anterior lobe 4 lines. 
The crown of the fourth premolar (p 4, figs. 11 and 14) is 1 inch (12 lines) in 
breadth, but sinks much deeper in the substance of the jaw before dividing. The 
narrow curved enamel-fold, answering to the island a in p s, is seen in the present 
less-worn tooth to be a fold marking off the hind boundary of the anterior lobe on 
the inner side of the crown : the circular island (b) is repeated ; the inner fold (c) of 
the hind lobe is wider. The grinding surface of the fourth premolar does not exceed 
that of the third in breadth. 
Only the anterior lobe of the first true molar (m i) is preserved ; it has been worn 
down two lines below the level of the last premolar, and the internal depression is 
wider and deeper than in the premolars. Its length is 1 inch 10 lines, of which more 
than 1 inch is implanted in the jaw, and it terminates below in a widely open pulp- 
cavity, fig. 13. 
The last three premolars and first true molar are in close apposition, one behind 
the other in the same line ; the anterior surface of one being pressed so close against 
the back part of the next as to have caused the disappearance of the enamel, if it 
had been originally laid upon the anterior surface of the crown so worn by pressure. 
The part of the jaw here preserved shows it to have been deep and narrow ; the 
hinder half probably resembled in its height that in the Nesodon ovinus. Two small 
foramina on the same horizontal line open upon the outer surface, midway between 
the upper and lower borders ; one of them 1 inch 9 lines behind the anterior end of 
the symphysis, the other 2 inches 3 lines behind the same part. The antero-posterior 
extent of the symphysis seems to have been not more than 2 inches. 
The breadth of the jaw at the alveoli of the first premolars is 1 inch 8 lines ; it 
becomes wider below, as well as in front and behind those teeth. 
The lower molars of the Toxodon, in their great length of the undivided crown, 
their breadth, their thinness, the narrow and prominent anterior lobe, and the broader 
but thinner and internally penetrated posterior lobe, manifest the same general type 
of structure, as those above-described, in the Nesodon-, the generic distinction is seen 
in the less complex disposition of the enamel. 
When the lower jaw and teeth of the Nesodon imhrlcatus are compared with those 
of the Nesodon ovinus, differences present themselves, which can only be referred, in 
the present state of zoological principles or philosophy, to a distinction of species. 
The crown of the outer incisor in Nesodon ovinus (Plate XVI. figs. 6 and 8, i s) is 
less than half the size of that in Nesodon imbricatus, so likewise is the crown of the 
canine and first grinder; and, supposing these teeth to be the deciduous ones, they 
