VEETEBRATA 63 



the posterior of each pair being mentioned first : first 

 pair, supramastoid and postfrontal ; second pair, squamosal 

 and postorbital ; third pair, quadrato-jugal and jugal 

 (or malar). The appearance of two foramina in this 

 roof determines the presence of two arches, a superior or 

 postorbito-squamosal, and an inferior, or quadrato-jugal. 

 The presence of one foramen only, results in the develop- 

 ment of a single arch. Which of the two arches remains 

 will depend on the position of the foramen, and the de- 

 gree of excavation of the inferior edge of the temporal 

 roof. In the Plesiosauria this arch is the quadrato-jugal ; 

 in the Lacertilia it is the postorbito-squamosal. In 

 the Theromora it is postorbito-squamosal, but much of 

 the quadrato-jugal adheres to its lower border, as in the 

 Mammalia. In the Archosaurian series (with two post- 

 orbital bars) these elements are widely separated. In the 

 Testudinata parts of both bones may be combined into one, 

 or one only may be represented. The supramastoid bone 

 disappears as a distinct element early in the history of the 

 Reptilia. In the Theromora the quadrate bone grows 

 shorter as we approach the Mammalia, and coincidentally 

 the quadrato-jugal disappears, as for instance in the Cyno- 

 dontia (Seeley). 



The quadrate bone is supported on a peduncle formed by 

 the transverse extension of the exoccipital, petrosal and 

 opisthothic bones. The opisthotic is generally early fused 

 with the exoccipital, but in the Ichthyopterygia and Testu- 

 dinata it is distinct, and takes the place of the petrosal as 

 a support of the quadrate in conjunction with the exoc- 

 cipital. In the Pythonomorpha a bone which occupies the 

 position of the terminal part of the opisthotic (or paroc- 

 cipital, which is the older name) issues from between the 

 exoccipital and petrosal, and supports the quadrate. 

 Whether this is homologous with part or all of the paroc- 

 cipital is an open question. For the present it is called, in 

 this book, the paroccipital, and it is probably a distinct 



