1 8 The Ganoids 



of the sturgeons. The general form is that of the sturgeon, 

 but the body is scaleless except on the upper caudal lobe, and 

 there are no plates on the median line of the skull. The oper- 

 cle and subopercle are present, the jaws are toothless, and there 

 are a few well-developed caudal rays. The caudal has large 

 fulcra. The single well-known species of this group, Chondrosteus 

 acipenseroides, is found in the Triassic rocks of England and 

 reaches a length of about three feet. It much resembles a 

 modern sturgeon, though differing in several technical respects. 

 Chondrosteus pachyurus is based on the tail of a species of much 

 larger size and Gyrosteus mirabilis, also of the English Triassic, 



FIG. 5. Chondrosteus acipenseroides Egerton. Family Chondrosteidoe. 

 (After Woodward.) 



is known from fragments of fishes which must have been 18 

 to 20 feet in length. 



The sturgeons constitute the recent family of Acipenserida, 

 characterized by the prolonged snout and toothless jaws and 

 the presence of four barbels below the snout. In the Acipen- 

 seridcz there are no branchiostegals and a median series of plates 

 is present on the head. The body is armed with five rows of 

 large bony bucklers, each often with a hooked spine, sharpest 

 in the young. Besides these, rhombic plates are developed 

 on the tail, besides large fulcra. The sturgeons are the youngest 

 of the Ganoids, not occurring before the Lower Eocene, one 

 species, Acipenser toliapicus occurring in the London clay. 

 About thirty living species of sturgeon are known, referred 

 to three genera: Acipenser, found throughout the Northern 

 Hemisphere, Scaphirhynchus, in the Mississippi Valley, and 

 Kessleria (later called Pseudoscaphirhynchus}, in Central Asia 

 alone. Most of the species belong to the genus Acipenser, which 

 abounds in all the rivers and seas in which salmon are found. 

 Some of the smaller species spend their lives in the rivers, ascend- 



