25 



The Carpus of Sphenodon with two centralia neither of which 

 appears in the borders of the carpus is usually regarded as a fair re- 

 presentative of the primitive reptilian carpus. The embryo carpus 

 of this type however has three centralia, the radial of which is exposed 

 on the side of the carpus, which is hence three rowed on the radial 

 and only two rowed on the ulnar side. 



In all described Therapsid carpi the condition of three rows on 

 the radial and two rows on the ulnar side is found, in Dimetrodon 

 Galechirus, Dicynodon, Theriodesmus, and others. In all these 



Fig. 2. 



Fig. 1. 



Fig. 1. Diagram of the supposed primitive tetrapod limb. The dotted line from 

 H to 4 is the axis, the branch dotted lines ending in 1. 2. 3. represent preaxial. 

 and that to 5 a postaxial radii. H=z humerus. JJ = ulna. R = radius. P= pisi- 

 form. 17' = ulnare. J = intermedium. Ä' = radiale. C". C". C". =' first second and 

 third centralia. D'-i», = first to fifth distal carpals. 



Fig. 2. Eudoskeleton of the pectoral fin of Eusthenopteron. A'-A'" = first to 

 third axial elements. 



cases the fourth and fifth distal carpals articulate directly with the 

 proximal row of the carpus. 



It will be at once noticed that this carpus is identical with the 

 tarsus we have already arrived at as the most primitive known Am- 

 phibian type, except for the presence of a pisiform. 



