STEREOGENTS CEOMERI. 297 



prefrontals {pr.f.) in a transverse suture. The prefrontals form the upper borders 

 of the orbits, and join an upward process of the maxillae to complete the orbits in 

 front. There were no nasals. Posteriorly the orbits are to a great extent cut oif 

 from the temporal fossa by a wall which, as in Podocnemis, seems to have been 

 constituted by the upturned edges of the palatines ijpl.) meeting processes of the 

 jugal and perhaps also in part by the postfrontal {po.f.). The premaxillse {pmx.) are 

 small ; they form the floor of the nasal opening, and appear to a considerable extent 

 on the palate, extending back to form the anterior boundary of the cleft [p-f.) which 

 runs back between the maxillae ; within and parallel to their sharp alveolar border 

 there is, on the palate, another slight ridge. The maxillse (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 

 palatine 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, g.), 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-Iaterally the pterygoid joins 

 the quadrate {g.), immediately internal to which is a large foramen for the internal 

 carotid (f.i.c). 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 (g.) difi'ers 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 



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