STEUEOGENTS CllOMERI. 
207 
prcfroutals {pr.f.) in a transverse suture. The prefrontals form the upper borders 
of the orbits, and join an u[)ward process of the maxilbe to complete the orbits in 
front. There were no nasals. Posteriorly the orbits arc to a great extent cut off 
from the temporal fossa by a wall which, as in Podocnemis, seems to have been 
constituted by the upturned edges of the palatines meeting processes of the 
jngal and perhaps also in part by the postfrontal The ])remaxillaD {pmx.) are 
small; they form the floor of the nasal opening, and appear to a considerable extent 
on the ])alatc, extending back to form the anterior boundary of the cleft {p-f.) wliich 
runs back between the maxillm ; within and parallel to their sharp alveolar border 
there is, on the palate, another slight ridge. The maxillm {mx.) form the greater 
part of the cutting-edge of the jaw and the anterior part of the palate ; in the middle 
line they are separated by the narrow cleft above referred to, but their edges converge 
backwards, while in Podocnemis there is a wide open space between them, increasing 
in width backwards. The palatines join the maxillae in an oblique suture ; their 
arrangement differs widely from that seen in Podocnemis. Instead of merely forming 
the posterior portion of the border of the internal nares, they here send in palatine 
plates, which seem to have met in the middle line forming a secondary hard palate, as 
in the mammals, and thus carrying back the opening of the internal nares to a point 
rather behind the middle of the skull. In the type and some other specimens these 
jDalatine plates do not actually meet, but are separated by a narrow cleft ; though 
probably this is due to slight distortion from pressure, since in one specimen at least 
they actually unite. This formation of a secondary hard palate and the consequent 
posterior position of the internal nares [i.n.) is correlated with the great width of 
the symphysis of the mandible : probably both the hard palate and the symphysis 
were covered with broad horny plates and were employed in crushing hard substances, 
such as the shells of Crustaceans or Molluscs. The form and relations of the palatines 
and internal nares in Stereogenys are shown in text-fig. 95. The pterygoids {pt.) join 
the palatines in an oblique suture and from the posterior ends of the palatine plates 
of the latter bones ridges run backwards and outwards towards the point of union of 
the former with the quadrates. The ectopterygoid processes {ec.pt.) are very large and 
together with the anterior upturned edge of the bones form the outer border of deep 
grooves (PI. XXV. fig. 1, ^.), which above lead into the orbits, into which they open 
immediately within the posterior wall above mentioned : the arrangement of these 
parts is almost exactly as in Podocnemis. Postero-laterally the pterygoid joins 
the quadrate {([.), immediately internal to which is a large foramen for the internal 
carotid {f.i.e.). In the middle line the pterygoids are to a considerable extent 
separated by the backward extension of the palatines, but behind this they meet 
for a short distance, probably covering the anterior part of the basioccipital. The 
quadrate {q.) differs from that of Podocnemis in being prolonged downwards, so that 
the mandibular articulation, which is strongly concave from side to side, is borne on a 
2 q 
