234 BONY FISHES vm. 2- 



however, only superficial and the plan of the fin is essentially actino- 

 pterygian. In the brain there is a thin pallium, thick corpus striatum, 

 and a valvula cerebelli. The pituitary is remarkable in that the hypo- 

 physial sac remains open to the mouth. In this and other features 

 (persistent pronephros) there are signs of neoteny. 



3. Order 2. Acipenseroidei 



The sturgeons are a rather isolated line descending from the palaeo- 

 niscids and characterized by reduction of bone. This was already 

 apparent in the Jurassic *Chondrosteus. Acipenser and other modern 

 sturgeons live in the sea but migrate up the river to breed. They may 

 reach a very large size (1,000 kg) and since a tenth of this is caviar 

 they are exceedingly valuable. They feed on invertebrates, which they 

 collect from mud stirred up from the bottom by a long snout. This is 

 flattened into a pear-shaped structure in Polyodon, the purely fresh- 

 water paddle fish of the Mississippi and in Psephurus in China. The 

 mouth of all sturgeons is small and the jaws weak and without teeth. 

 In Polyodon there is a filtering arrangement of gill-rakers in the 

 pharynx. The jaws of sturgeons hang free from the hyomandibular 

 and symplectic, and can be swung downward and forward during 

 feeding. The skull and skeleton is almost wholly cartilaginous and the 

 dermal skeleton much reduced. The tail is covered with rhomboidal 

 scales, but on the front of the body there are five lines of bony plates 

 bearing spines, with the skin in between carrying structures similar to 

 denticles. There is an open spiracle. The internal anatomy of the 

 sturgeons shows various features that have been held to show affinity 

 with the elasmobranchs ; for instance, besides the spiral valve there is 

 a conus arteriosus in the heart and a single pericardio-peritoneal canal. 

 However, there can be no doubt that they are descended from an 

 early offshoot from the actinopterygian line. They retain some fea- 

 tures lost by most members of the line, but resemble the Teleostei in 

 other characters, for instance a thin roof to the cerebral hemispheres. 



The palaeoniscids and sturgeons may be grouped together in a 

 Superorder Chondrostci and placed with them is a third Order Sub- 

 holostei, probably a mixed group, including forms that resemble 

 palaeoniscids, but show various trends towards the holostean grade 

 of organization i*Ptycholepis). 



4. Superorder 2. Holostei 



During the later Permian period the palaeoniscids gave rise to 

 fishes of a different type, which replaced their ancestors almost com- 



