67 
In the Stegocephali, there are usually, as I understand FrirscH 
(No. 15), the only extensive work on the subject that I have at my 
disposal, two dermo-occipitals, or supraoccipitals, one on each side of 
the head; but these two bones may, according to Macer (No. 22), 
be found fused with each other to form a single median bone, or 
they may be fused with the epiotic bones, or with the parietals, or 
in still other combinations. In Tremotosaurus both the supraoccipitals 
and the epiotics are said by Baur (No. 6) to be traversed by a 
lateral canal which he considers as the homologue of the supra- 
temporal crosscommissures of both Amia and Polypterus. The main 
lateral canal on each side, in the Stegocephali, does not, however, 
seem to traverse the epiotic, as it does the assumed homologue of 
that bone im Amia and Polypterus, being interrupted by the external 
opening of the ear. Because of their relations to the commissural 
canal, the epiotic and supraoccipital bones of each side of the head of 
the Stegocephali are considered by Baur as the undoubted homologues 
of the three so-called supratemporal bones of Polypterus and of the 
single so-called extrascapular of Amia. The commissure in the 
Stegocephali may, however, as stated above, be the homologue of the 
commissural canals of the Characinidae and certain other Teleosts, and 
hence of the middle head line of pit organs of Esox, in which case the 
supposed homologies of the related bones would necessarily be greatly 
changed. All speculation regarding it is certainly useless until the 
complete distribution of the sensory lines, and their innervation, in 
the cartilaginous Ganoids and the Dipnoids, and perhaps also in living 
Amphibia, is definitely known. 
Mager has recently attempted to establish the homologies of the 
bones here concerned by comparison with embryological conditions 
that he finds in man. In his preliminary communication (No. 20) and 
in his own résumé of his final work (No. 21), he refers to the supra- 
occipitals and epiotics of the Stegocephali as the retroparietals, and 
he says that they are represented in human embryos by four centres 
of ossification, from which may develop, in the adult, four distinct 
bones, the interparietals. In Polypterus the homologues of these so- 
called interparietal bones are said to be “les quatre plaques osseuses 
postérieures, en série transversale, de ce qu’on appelle bouclier osseux 
sus-occipital”, and these osseous plates are said to be shown by 
WIEDERSHEIM in what must be Figure 77, page 108, in the 1893 edition 
of his Grundriß. They there form a transverse series immediately 
posterior to the six supratemporal bones of TraqQuarr’s descriptions 
of the fish, and are said by that author to be simply “proper scales 
5* 
