500 CHONDEOSTEI [CH. 



In the skull there is no supra-occipital, but two supratemporals 

 occupy its place. The upper jaw is short and normal, i.e. resembles 

 that of Teleostei, but the lower jaw has the splenial bone and in 

 addition there is a dermal bone, the gular plate, on the under side 

 of the throat between the halves of the jaw. The air-bladder is like 

 that of Lepidosteus, and Amia appears to use it as an organ of 

 respiration when the water becomes foul. 



Numerous fossil fish belong to the Protospondyli, and in most 

 cases their pre- and postcentra do not -even form rings, but oblique 

 wedges, and no complete centra are formed even in the trunk. 



Order IV. Chondrostei. 



The Chondrostei or Sturgeons include four genera divided 

 into two families. They agree with the two preceding orders in 

 possessing the archaic features of vasa efferentia, conus, optic 

 chiasma and spiral valve, and the oviducts are short and open by 

 wide funnels into the coelom. 



The Chondrostei are distinguished by the condition of the 

 vertebral column. As in the two preceding orders, dorsal and 

 ventral intercalaries are present as well as neural and haemal 

 arches, but only the neural and haemal arches Become ossified, and 

 that incompletely ; the notochord is invested with an unsegmented 

 cartilaginous sheath, which is a modification of its own sheath. No 

 centra are formed. A large number of the arch-pieces are fused into 

 a cartilaginous tube and amalgamated with the great cartilaginous 

 cranium. The tail fin is typically heterocercal, and the front edge 

 of the dorsal lobe is covered with a line of fulcra. The rostrum 

 of the skull is produced into a great prae-oral snout which is 

 used for stirring up the mud in which the fish finds the worms 

 on which it feeds. The jaws are feeble, devoid of teeth and 

 attached only to the skull by the hyoid arch. It is an interesting 

 fact that not only the hyomandibular cartilage, but also some of the 

 pharyngobranchials, articulate with the cartilaginous skull. The 

 spiracle usually persists and has a rudimentary gill (pseudobranch) 

 on its anterior wall. Bones replacing the cartilage are very feebly 

 developed. In very old fish parethmoid and orbitosphenoid bones 

 may appear, as well as pro-otics and opisthotics. In the upper 

 jaw there is always a palatine bone and sometimes ectoptery- 

 goids, metapterygoids and quadrates, but the cartilage is merely 

 invested it still persists. In the second arch there is a 



