GANOCEPHALA. 197 



verting the notch into a foramen. This forms a wide oval, 

 the apex being produced into a long spine ; but towards the 

 end of the tail the spine becomes shortened, and the hseinal 

 arch reduced to a mere flattened ring. 



The size of the canal for the protection of the caudal blood- 

 vessels indicates the powerful muscular actions of that part, 

 as the produced spines from both neural and hsemal arches 

 bespeak the provision made for muscular attachments, and 

 the vertical development of the caudal swimming organ. 



The skull of the Archegosaurus appears to have retained 

 much of its primary cartilage internally, and ossification to 

 have been chiefly active at the surface ; where, as in the com- 

 bined dermo-neural ossifications of the skull in the sturgeons 

 and salamandroid fishes — e.g., Polypterus, Amice, Lepidosteus — 

 these ossifications have started from centres more numerous 

 than those of the true vertebral system in the skull of saurian 

 reptiles. This gives the character of the present extinct order 

 of cold-blooded, air-breathing animals. 



The skull is much flattened or depressed, triangular, with 

 rounded angles, and the front one more or less produced 

 according to the species ; and in some species according to 

 the age of the individual. The super-occipital (fig. 65, 4), is 

 represented as in the salamandroid fishes, by a pair of flat 

 bones ; the pair external to these, and forming the prominent 

 angles of the occipital region, represent the " par-occipitals." 

 The lower peripheral surface of the basi-sphenoidal cartilage 

 is ossified with a concave border towards the notochord behind, 

 to the capsule of which it seems to have been attached. The 

 alisphenoids were doubtless cartilaginous, and the protocra- 

 nium there unaltered, as it was apparently in the ex-occipital 

 region. The peripheral ossifications above representing the 

 "parietal" (7), form a pair of oblong flat bones, with the 

 " foramen parietale" in the mid- suture. External to these, and 

 wedged between the parietals, the super- and par-occipitals, 



