26 



Comparison of a large series of limbs will shew that in all pri- 

 mitive types the carpus or tarsus is long compared with the forearm 

 or leg and the metapodials and that in the higher types it tends 

 to become progressively shorter. This change is of importance as 

 suggesting that the carpals and tarsals are really bones of the same 

 order as the other bones of the leg. This conclusion is strengthed 

 by the fact that it is not uncommon for a distal carpal to take on a 

 close resemblence to the form of a metacarpal. 



It is certain that the Tetrapods have been derived from some 

 fish, and that this fish must have been a bony fish in existance in 

 Devonian times. The only bony fish known at this time are the Dipnoi, 

 the ,,Crossopterygii", and the Palaeoniscids. The latter group may 

 be at once excluded from consideration, because their paired fins 

 are so advanced that their main support is provided by dermoskeletal 

 elements and it is obvious that the ancestral tetrapod must have 

 had a large endoskeleton in its paired fins. We are thus left with 

 the Dipnoi and the ,,Crossopterygii". Amongst these types only 

 two types of fin skeleton are known: 



1. The long biserial archipterygium found in Ceratodus, which 

 is apparently the primitive type, as it alone is common to the two 

 orders and the ,,Crossopterygian" family with long fins is the first 

 to become extinct. 



2. The short reduced archipterygium found in Eusthenopteron 

 and Rhizodopsis. This fin is supported by an endoskeleton con- 

 sisting of an axis of four elements, the first second and third of which 

 each carry a preaxial radial, the third having also a postaxial bone. 

 It is from such a fin that I believe the tetrapod limb to have been 

 derived. 



If the primitive limb as described above be examined it will be 

 found that if the axis is assumed to run along the humerus, ulna, 

 ulnare, and fourth finger, there are three preaxial and one postaxial 

 radii; the first, bourne by the humerus, running through the radius, 

 radiale, third centrale, first distal carpal, and first digit. The inter- 

 medium, second centrale, second distal carpal, and second digit 

 form the preaxial radial bourn by the second axial element, the 

 ulna; the first centrale, third distal carpal, and third digit form 

 the preaxial, and the fifth distal carpal and fifth digit the post axial 

 radii of the third axial element, the ulnare. The pisiform is perhaps 

 the remnant of the post axial radial of the second axial elements. 



