662 



opercular canal. The suborbital canal crosses the squamosal, post- 

 orbital, jugal and maxillary ; the supraorbital the postfrontal, lacrimal 

 and nasal; the supratemporal cross-commissure the supraoccipital 

 and paroccipital plates. The fourth canal is situated on the pro- 

 quamosal and runs forwards to the maxillary. There is no doubt, 

 that the elements traversed by the supratemporal cross-commissure 

 in the Crossopterygii and Amiadae and Stegocephali are homologous. 

 We reach therefore the result that the supratemporal (extrascapula) 

 of Amia, which in Polypterus is represented by three lateral ele- 

 ments is homologous to the supraoccipital- and paroccipital plate in 

 the Stegocephali. The discovery of a skull of Keraterpetou in the 

 Carboniferous of England, lately described by Mr. C. W. Andrews 

 of the British Museum, makes it probable that the paroccipital plate 

 (epiotic) may also contain the posttemporal (suprascapula). Here the 

 cross-commissure is very well seen, the paroccipital plate sends a long 

 process backwards and this was probably connected with the shoulder- 

 girdle by ligament (Andrews). 



In Amia, Polypterus, Osteolepis, the praeoperculum is pierced 

 by the operculo-mandibular canal, in the Stegocephali this canal is 

 found in the prosquamosal. Therefore the prosquamosal of the Stego- 

 cephali is homologous to the praeoperculum. The quadratojugal may 

 possibly represent the subopercular, but concerning the fate of the 

 opercular nothing can be ascertained. It is possible however that in 

 some early Crossopterygians the diflferentation into different operculars 

 was not yet etablished and that from such a form the Stegocephali 

 took their origin. That such Crossopterygians formerly existed, is 

 shown by the family Coelacanthidae with the single operculum on 

 each side. 



The vertebral column. 



If we examine the vertebral column of Archegosaurus, we 

 see that the notochord is still developed and that in the dorsal region 

 each body of the vertebra consists of three parts, two lateral ones, the 

 pleuro centra, and one inferior one, the intercentrum (hypocen- 

 trum). In the tail region we find even five elements; the two pleurocentra 

 and below them two small haemacentra (hypocentra pleuralia), and 

 the intercentrum to which the haemal arches (chevron bones) are at- 

 tached. Two modifications of these primitive conditions take place. In the 

 dorsal region the three elements may become enlarged and in the tail 

 by their extension two complete rings may be formed; each segment 

 being represented by two vertebral rings. Such a condition we see 

 in Cricotus and Diplovertebron. Or the intercentrum may be 



