ZOOLOGY. 211 
riorly. This raised margin is turned inwards above the postfron- 
tals, giving the orbits a slight postero-superior notch, which igs 
much less developed than in Pt. declivis, according to Owen’s 
description: It is slightly Tugose in consequence of transverse 
grooves. They are prolonged into the prefrontal tuberosities, 
which are very large, more developed than in any other species, 
resembling rudimental horns. They present a sharp edge out- 
wardly, as the front margin of the orbits and the superior and 
anterior planes are at right angles to each other. The middle 
line of the front, descending more gradually, causes the angle 
between it and the premaxillary to be rather more open. 
The premaxillary region is remarkably contracted ; and its length 
from the front is about equal 
to the distance between the 
prefrontal horns, producing 
a T-shaped outline. On 
the middle line it presents 
a high laminar keel, which 
separates two parallel sulci. 
These extend to the end of 
the muzzle, and are bound- 
ed externally by a strong 
longitudinal angle. The 
external face of the maxil- 
lary is occupied by a wider 
longitudinal concavity par- 
allelto the last. The poste- Fig. 8. — Lystrosaurus frontosus. 
rior angle of the bone flares (Lettering as in fig. 1.) 
out behind it. The posterior (superior) “spine” of the premaxil- 
lary extends far between the nasals, and nearly to the anterior pro- 
longation of the frontal. 
The nasals are prominent, each presenting a low boss forwards, 
which enclose a concavity on each side with the tuberosity of the 
premaxillary spine. They overhang the nares superiorly. 
The lachrymal is a small bone intercalated between the pre- 
frontal and the maxillary. In front of and below it, a larger bone 
extends to the nostril, constituting the principal part of its poste- 
rior boundary. This bone is described by Owen in the Pt. lati- 
frons. Its homologies are not determined. 
The alveolar margin of the upper jaw is undulating, presenting 
a short median beak-like prominence, then a concavity, and poste- 
