IN THE REPTILIA. 21 



oped, but the primitive roof has suffered diminution by absorption from the inferior 

 edge, or from both the inferior and the posterior edges. In Chrysemys, where a bar 

 has been produced (PL Y, Fig. 5), it consists of the unseparated zygomatic and supra- 

 temporal, the anterior elements of which are the jugal and postorbital ; and the poste- 

 rior, the zygomatic and supratemporal. The supramastoid is wanting even in the 

 genera (Hydraspis e. g?) with a parietoquadrate arch. The postfrontal and post- 

 orbital are not distinguished. In genera, where the posterior excavation is very deep 

 (e. g., Trionyx), the connection between the postorbital and the supratemporal is 

 interrupted, and a zygomatic arch remains (PI. Y, Pig. 6). It was comparison 

 of this type with the Lacertilia that led Baur* to conclude that the bar of the 

 latter order is the zygomatic. 



The Saueopteetgiia possess but a single arch, and this is the zygomatic accord- 

 ing to the description of !Nothosaurus given by Yon Meyer. The supratemporal 

 has no anterior connections according to this author, and the supramastoid is not 

 described. From all that I can gather from Owen's descriptions and figures of 

 Plesiosaurus the structure is the same ; which is confirmed by observation on such 

 imperfect specimens as are accessible to me. The postfrontal is not continued above 

 the large temporal foramen; nor is the postorbital continued posteriorly. In the 

 latter point the structure differs from that of the Theriodonta. The type of the 

 Sauropterygia may be derived from that of the Theriodonta by the extension of 

 the infratemporal foramen upwards to the parietal bone, thus cutting off the posterior 

 connections of the postorbital and postfrontal bones. In this respect this type 

 resembles the Testudinata (PI. Y, Fig. 4). 



The Mesozoic reptiles (including the existing orders) present us then with the 

 following types of postorbital structure : 



I. One foramen ; generally a zygomatic arch. 



No supramastoid bone ; postfrontal and postorbital fused ; a paroccipital Tesludinata. 



Postfrontal and postorbital distinct Sauropterygia. 



II. A supramastoid foramen only. 



Supramastoid and zygomatic arches not distinguished from each other ; a paroccipital, 



lehthyopterygia. 



III. A supratemporal and infratemporal foramina. 



Supramastoid and zygomatic arches Crocodilia. 



Dinosauria. 



Pterosaur la. 



Hhynchocephalia. 



* Amer. Journal of Morphologj^ 1889, p. 473. 



