PAIRED LIMBS 555 



In the modern actinopterygians there is only one set of bony 

 elements, probably radials, and these have sunk into the body- 

 wall so that the rays make the sole external support for the fins. 

 All such fish-fins are included in the general term ichthyo- 

 pterygium, and from this is 

 derived the cheiropterygium 

 of the tetrapods, a generalised 

 diagram of which is shown 

 in Fig. 331. There is no 

 direct fossil evidence of the 

 change-over, but it is not 

 unreasonable to believe that 

 the basals became the 

 humerus or femur, and the 

 radials the other bones. In 

 the nomenclature of those 

 which form the wrist and 

 ankle there is unfortunately 

 much confusion, human 

 anatomists favouring one set 



B 



Fig. 432. — Placoderm Fishes. — 

 From Swinnerton. 



(a) Clitnatius (Devonian) ; [b] Acanthodes 

 (Permian). 



Both belong to the Order Acanthodii, 

 v.hich had a strong spine in front of 

 each fin. Climatius, one of the oldest 

 known fishes, has on each side a row of 

 small fins from the pectoral to the 

 pelvic. 



Fig. 433. — Right forehmb (A) and hind- 

 hmb (B) of Ichthyosaur. — After 

 Lydekker. (Guide to Fossil Birds, 

 Reptiles, etc., British Museum [Natural 

 History], I934-) 



c>, c2, Centrale ; /., fibula ;/•., fibulare ; h., humerus; 

 J., intermedium ; r., radius ; ri., radiale ; /., tibia; 

 /!., tibiale ; u., ulna ; m'., ulnare. 



of names and zoologists another ; the chief variants are shown in 

 Table VI. The primitive number of digits is probably five in each 

 limb, but there are no modern Amphibia and few Stegocephalia 

 with the full set of twenty. The hind-Hmb of the frog is specialised 

 for both jumping and swimming, and has very long tibiale and 

 fibulare. Both limbs show a feature of no obvious adaptive 

 significance, which is found also in many other vertebrates — 



