52 PROFESSOR HUXLEY ON THE OSTEOLOGY OE THE GENUS GLYPTODON. 
are inclined a little inwards as well as upwards, reversing the direction of the grinding- 
faces of the upper teeth. 
The mandibular teeth seem to have been nearly straight, without either external or 
internal concavity. Their long axes are inclined rather backwards as well as downwards. 
The alveolus of the fourth tooth, on the right side, is laid open ; and I judge from it 
that the fourth tooth must have had a length of about 3^ inches ; and the others might 
have had the same dimensions, except the first, which is certainly shorter, probably not 
exceeding 2^ inches. 
A considerable canal traverses the right ascending ramus from behind and below, up- 
wards, forwards, and outwards. Its external aperture, oval, 0*3 inch wide, lies upon 
the outer face of the ramus, on a level with the alveolar margin, and rather nearer its 
anterior than its posterior edge (Plate V. fig. 2). The inner end of the canal, which is 
T7 inch long, terminates in the broken cancellous structure, on the outer side of what 
appears to be the remains of the last alveolus. 
I cannot certainly discern any remains of a corresponding canal in the left ascending 
ramus. 
All that remains to be described in this skull is a fragment of the basis cranii, con- 
sisting of part of the anchylosed basi- and pre-sphenoid bones. The presphenoid (Plate 
VI. fig. 2) is remarkable for the strong crest or spine into which the middle of its upper 
surface is produced, and which was not improbably continued into an ethmoidal crista 
galli. The posterior apertures of the passages for the optic nerves are ellipses, with their 
long axes directed upwards and outwards ; they are about a quarter of an inch in dia- 
meter, and are continued into two canals, which are traceable, outwards and upwards, 
for about an inch in the substance of the orbitosphenoids. On each side, below and 
external to the optic foramina, are strong grooves which formed the inner portion of the 
confluent foramen rotundum and sphenorbital fissure. The front face of the presphenoid 
and the roots of the orbitosphenoids are excavated by deep sphenoidal sinuses. 
Comparison of the Skull of the present specimen with that of the typical Glyptodon 
clavipes. — The principal parts which exist in both skulls, and may therefore serve as 
terms of comparison, are, 1, the nasofrontal region of the roof of the skull; 2, the 
descending zygomatic processes ; 3, the alveoli ; and 4, the basi- and pre-sphenoid. 
1. The resemblances in size and general configuration between the nasofrontal regions 
of the two skulls are so obvious that I need hardly dwell upon them at any length. 
The present specimen differs from the type in the more rounded contour of the nasal 
bones, in the persistence of the nasal suture, in the less rugosity and squareness of the 
supraorbital prominences, and in the far less marked definition of the temporal ridges ; 
but none of these characters appear to me to have more than an individual import- 
ance, and I am inclined to suspect that they depend largely on the less advanced age 
of the present specimen. 
2. The zygomatic processes have the same length (measured from the infraorbital fora- 
men) in each case. They are slightly narrower in the type specimen ; in other respects 
