GREGORY, PRESEXT STATUS OF OR[GIX OF TETRAPODA S31 



1). The rostro-f rental segment has usually become more elongate and 

 always more completely differentiated (nostrils larger, separate nasals 

 and premaxillae, distinct internal nares). 



2). The orbits have increased in size and are generally displaced back- 

 ward farther from the nares. 



3 ) . A pineal foramen, between the parietals has appeared. 



4). The elements of the skull-roof usually have acquired more polyg- 

 onal, or angulate, and less rectangular, outlines. 



5). The postorbito-pterotic series, lying above the spiracle and bear- 

 ing a supraorbital sensory tract has given rise to the postorbital, inter- 

 temporal and true supratemporal (pterotic) of the Stegocephalia (cf., 

 St eg ops divaricata Moodie). 



6). The circumorbital series have come into closer sutural relations 

 with the surrounding elements and appear as the prefrontal, lacrymal, 

 jugal, postorbital and postfrontal. 



7). When the shoulder-girdle became freed from the skull by the 

 atrophy of the posttemporals the median nuchal plate became firmh- 

 attached to the parietals and was divided by a median suture into the 

 paired dermosupraoccipitals, vv^hile the lateral nuchals, also becoming 

 attached to the parietals and pterotics, gave rise to the tabularia (epi- 

 otics). 



8). The operculum and suboperculum, as well as the median paired 

 and lateral gulars, lost their bony constituents and became membranous. 

 This may have resulted from a complexity of changed conditions follow- 

 ing upon the assumption of air-breathing habits, the dwindling of the 

 branchial arches and the reduction of the cleithrum (with which in 

 Crossopterygii the operculum is in close contact, see page 354). The 

 iDranchial chamber may also have served more or less as a resonating 

 chamber and sound vibrations may have been transmitted from the outer 

 air, through the operculum and hyomandibular (stapes)- to the side of 

 the otic capsule, so that the operculum may have given rise to the tym- 

 panic membrane of Stereospondyli, which had the locus of the operculum 

 and was in contact with the squamosal (preoperculum), stapes (hyo- 

 mandibular) and tabulare (lateral shoulder plate). 



9). The preoperculum and interoperculum (X') covering the side and 

 lower part of the quadrate in Rhipidistia did not share the fate of the 

 operculum but gave rise to the squamosal and quadrato jugal, respectively. 

 10). As a result of the loss of the bony character of the opercula the 

 fossilized skull of the Stegocephali is sharply truncated behind the occiput 

 and squamosal and in the young stages the branchial arches (cf., Branch- 

 iosaurus) are exposed. As the Stegocephali were almost certainly de- 



