SKULL. 133 



the conical cavities of which contain a part of the remains of the 

 notochord. The notochord as a rule persists also in the centre of 

 the vertebral body as a thin cord (connecting the dilated inter- 

 vertebral portions, fig. 570 a). In the bony Ganoids and the 

 Teleosteans the biconcave* vertebral bodies are completely ossified 

 and fuse with the corresponding upper and lower bony arches, so as to 

 form a complete vertebra. In some parts of the trunk ribs are attached 

 to the pieces of the ventral arches (hsemapophyses) which here diverge 

 from one another; and there are often in addition ossifications of the 

 inter-muscular ligaments. 



The structure of the skull in Fishes presents a series of grades of 

 development culminating in the complicated skull of the Teleostei. 

 The primordial skull of the Cydostomes is the simplest. It consists 

 of a cartilagino-niembranous cranial capsule, in the hard basilar part 



FIG. 5S5. Cephalic skeleton of the Sturgeon (after Wiedersheim) . Eo, rostrum ; Cn, nasal 

 pit; O, orbit; Hm, hyomandibular ; S, symplectic ; Pq, palatoquadrate ; Md, lower jaw 

 Hy, hyoid bone ; V, foramen for the vagus ; R, ribs. 



of which the notochord ends. Two bony capsules lateral appendages 

 of the bony basilar region enclose the auditory organ, while two 

 anterior pieces are connected with the complicated apparatus of the 

 facial and palatal cartilages. The primordial skull of the Selachians 

 (fig. 571) shows a further advance in development. It has the form 

 of a simple cartilaginous capsule which is not further divided into 

 separate pieces. The notochord ends in its base. In the sturgeon 

 (fig. 585), there are bony pieces as well as the cartilaginous cranial 

 capsule. These consist of a flat basilar bone the parasphenoid and 

 a system of dermal membrane bones. A true bony cranial invest- 

 ment is first developed round the primordial skull of the Dipnoi. In 

 the bony skulls of the Ganoidei and Teleostei there still remain 

 continuous portions of the primordial cartilaginous cranium (Pike 



* In the genus Lepidosteus alone is there an anterior articular surface on the 

 vertebral bodies ; the centra being convex in front and concave behind. 



