18 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, 
SS 
: MS) MY ex ° 
WY 
Fig. 5.—Chondrosteus acipenseroides Egerton. Family Chondrosteide. 
(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 Actpenseride, 
characterized by the prolonged snout and toothless jaws and 
the presence of four barbels below the snout. In the Actpen- 
serid@é 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, Acitpenser toliapicus occurring in the London clay. 
About thirty living species of sturgeon are known, referred 
to three genera: Acztpenser, found throughout the Northern 
Hemisphere, Scaphirhynchus, in the Mississippi Valley, and 
Kesslerta (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- 
