570 PISCES—FISHES. 
oral part of the hypophysis retains its opening into the mouth, The 
genus Calamoichthys has very similar characters, but no pelvic fins. 
These two forms may almost be called living fossils. 
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Fic. 304.—Larva of Polypterus (after Budgett), 14 inch in length, 
e.g., Large external gill of the hyoid arch; Pc., pectoral fins; Pv., pelvic 
fins. ‘The larva is drawn in a very characteristic attitude. 
The following three orders are often grouped as Actino- 
pterygii, with the following characters. The paired fins are 
never lobate, they have short basal pieces, and are mainly 
supported by dermal fin-rays. 
Order 2, CHONDROSTEI—Wwith cartilaginous 
internal skeleton 
Living examples :—Sturgeon (Acipenser), Polyodon, Sca- 
phirhynchus. 
Fic. 305.—Sturgeon (Aczpenser sturio). 
Note the elongated snout, the barbules bounding the ventral mouth, 
the operculum covering the gills, the rows of bony scutes, the 
markedly heterocercal tail. 
Extinct examples :— Chetrolepis, Pal@oniscus, Chondrosteus. 
In the sturgeon (Acépenser) the skin bears five rows of large bony 
scutes; the tail is heterocercal; the notochord is unsegmented. A 
snout, with pendent barbules, extends in front of the ventral mouth, 
which is rounded and toothless. Sturgeons feed on other fishes, 
