SKULL — GANOIDS 



99 



Chondrostei. — Except in old individuals there are very few 

 ossifications of the chondrocranium in Chondrostei, these being, 

 probably, ectethmoids, orbito- and ahsphenoids and prootics. The 

 cranium passes behind into the vertebral column, the dorsal arches 

 of which are fused . The roof is complete, with the exception 

 of a small posterior fontanelle, and brain case and nasal capsules 

 are not sharply separated, although the brain reaches forwards only 

 to the orbital region. There is a long rostrum; the nares are far 

 forwards, the nasal capsules being at the base of the rostrum. The 

 barbels on the rostrum are supported by cartilage 

 rods. Six vertebrae are fused to the hinder end 

 of the cranium. 



The numerous dermal bones of the roof of 

 the adult skull are separated from the cartilage 

 by perichondrium. A few of these (figs. 105, 

 106) are readily compared with those of other 

 Vertebrates, among them dermoccipital, 

 supracleithra, parietals, frontals, postorbitals, 



Fig. 105. Fig. 106. 



Fig. 105. — Side view of skull of Scaphirhynchus. d, clavicle, and cleithrum; 

 do, dermoccipital; /, frontal; w, naris; nu, nuchal; op, operculare; p, parietal; pf, 

 post-frontal; pr, prefrontal; sc, supracleithra; so, suborbitals; sq, squamosal; st, 

 supratemporal. 



Fig. 106. — Skull of Acipenser sturio, the outline of the chondrocranium stippled 

 (Gegenbaur, '98). do, dermoccipital; /r, frontal; n, nasal; nu, nuchal; op, operculare; 

 pa, parietal; pf, prefrontal; pof, postfrontal; sc, supracleithrum; sq, squamosal; st, 

 supratemporal. 



postfrontals and squamosals. The median series is continued 

 backwards by an unpaired nuchal bone behind the dermoccipi- 

 tal. The ventral side of the cranium has a large parasphenoid 

 extending from beneath the anterior vertebrae almost to the tip of 

 the skull, a part of its anterior end being included in the cartilage 

 cranial floor. It gives off a transverse process on either side which 

 extends up under the postorbital region. 



The jaws (fig. 107) are very primitive, consisting of pterygo- 

 quadrate and Meckehan and are connected with the cranium by 



