172 



SUB-CLASS GANOIDEI. 



attached to one another in much the same way and are 

 arranged in oblique rows. They are rhombic in form 

 and similarly covered by ganoin. The canals in the 

 deeper part of them, containing blood vessels, open on the 

 surface, but differ from those of Lepidosteus in being branched. 

 They are without spines and the remains of spines, except at 

 the bases and on the hinder surface of the pectoral fins and on 

 the plates overlying the shoulder girdle. They are covered by 

 soft skin in the adult, but this is frequently rubbed off in pre- 

 served specimens, leading to the view 

 that the scales are freely exposed in 

 the adult. 



The vertebral column is completely 

 ossified ; the vertebrae are amphi- 

 coelous, as in Teleosteans, and carry 

 in the body two pairs of ribs, of which 

 the shorter and ventral pass between 

 the muscles and the peritoneum and 

 correspond to the ribs of other forms, 

 while the dorsal and larger pair lie 

 between the dorsal and ventral lateral 

 muscles. The body of the first verte- 

 bra is united to the skull, but the 

 arch and ribs are separate. The 

 cranium consists mainly of persistent 

 cartilage (Fig. 98) with a large fon- 

 tanelle in the roof and in the floor 

 (pituitary) and a few cartilage bones, of 

 which the most important are exoc- 

 cipitals which completely surround 

 the foramen magnum, opisthothic (~), 



sphenoid (? sphenethmoid), postfrontal (8), prefrontal (9) and 

 a median ethmoid (^). There is a slender arcade of car- 

 tilage passing forward from the postfrontal to the anterior part 

 of the sphenoid cartilage. 



The whole is invested dorsally by membrane bones, while 

 ventrally there is a parasphenoid which extends back beneath 

 the body of the first vertebra, and two vomers (as in Lepidos- 

 teus). The mandibular arch is very like that of Teleosteans : in 

 the upper jaw arcade (Fig. 100) are palatine, ectopterygoid, 



-7 



FIG. 98. Dorsal view of the 

 u cartilaginous cranium of 

 Polypterus with the mem- 

 brane bones removed (after 

 Traquair).- 1 Ethmoid, 2 

 nasal opening, 3 sphenoid, 

 4 optic foramen, 5 occipital 

 bone, 6 foramen magnum, 

 7 opisthotic, 8 post-frontal, 

 9 pre-frontal. 



