20 POSTERIOR CRAXIAL ARCHES 



frontal. I cannot agree with Baur that this arch in the lizards is the zygomatic arch 

 of the other Reptilian orders. The supramastoid bone is, in the Lacertilia, wanting, 

 but whether by atrophy or by fusion with the parietal, forming the supramastoid pro- 

 cess of the latter, I do not know. An element intervenes between the supratemporal 

 bone and the parietal above and the exoccipital within, which Dr. Baur regards as 

 the supratemporal. With this I do not agree, and for the following considerations: 

 In neither adult nor young Lacertilia is there present any other element which 

 can be regarded as the homologue of the paroccipital of Ichthyosaurus, the Testu- 

 dinata and Dinosauria. In the Pythonomorpha this element is deej)ly embraced be- 

 tween the petrosal (prootic) and exoccipital, precisely as is the paroccipital (Fig. 3). 

 In the Lacertilia it is carried on the extremity of these elements. Moreover the 

 supramastoid is a purely roof-bone, and has no connection primitively with the 

 petrosal, and very little with the exoccipital. It cannot be identified with the supra- 

 temporal because it exists contemporaneously with that element in Ichthyosaurus,* as 

 well as in the Cotylosaurian genera Chilonyx and Pariotichus above described. I 

 therefore maintain the homology of this bone with the paroccipital as I presented it 

 in my paper of 1870, where I used for it Huxley's term "opisthotic." (PI. IV, Pig. 5). 

 Parker, in his paper on the Development of the Skull in the Lacertilia,f did not dis- 

 cover a distinct ossification in the position of paroccipital, although he finds a portion 

 of the exoccipital marked off by a shallow groove, which he calls opisthotic. The 

 true paroccipital he calls the " second supratemporal." 



In the Ophidia there is no zygomatic or supratemporal arch, and the supratem- 

 poral as well as the supramastoid bones have disappeared. The paroccipital is the 

 only one of the suspensors of the quadrate remaining. This element had been gen- 

 erally homologized with the " squamosal " (supratemporal) by authors, but in my 

 paper of 1870 I identified it with the paroccipital of the Lacertilia (" opisthotic ;" 

 supratemporal of Baur), with which Baur agrees. In the more specialized snakes its 

 squamosal attachment to the cranial wall resembles that of the squamosal bones of 

 higher Vertebrata, and its general position is that of that element. When, however, 

 the lower snakes, e.g., llysia, are examined, it is found to have the same position in 

 the embrace of the exoccipital and petrosal bones, as in the Pythonomorpha, and to 

 be clearly homologous with that element which I have thought to be the paroccipital 

 (PI. ly. Fig. 0). 



In the Testudij^ata, as pointed out by Baur, no foramina have been devel- 



*The process of the parietal which joius tlie supramastoid arch in Diclonius (Plate III) may represent the supra- 

 temporal. 



t Philosophical Transactions Royal Soc, 1879, p. 631. 



