416 H. B. POLLARD, 



again seen that the best for the purpose is Chlamydoselachus. The 

 shouldergirdle in the former is considerably reduced but shows the 

 same essential character even to the shape of the foramen for the 

 nerve and bloodvessel. The propterygium is smaller in Chlamydo- 

 selachus. In neither does it appear to bear rays. The mesoptery- 

 gium in Chlamydoselachus still articulates with the shoulder girdle. 

 On carefully removing the muscles from a Polypterus fin several nerve 

 foramina or foramina for vessels are seen and also superficial indi- 

 cations of a former division of the apparently homogeneous part of 

 the cartilage into a number of blocks exactly as in Chlamydoselachus. 



The distal partially ossified rays of Polypterus correspond to the 

 2 distal rows of Chlamydoselachus and in both animals the second 

 row shows an alternate arrangement with the first. In the smallest 

 Polypterus, whose fin was cut in horizontal sections a third very 

 small row was present alternating with the second, that is in line 

 with the first row. Very curious is the existence of a small cartilage 

 in front of the propterygium and applied to it at two thirds of its 

 length distally (Wiedersheim). 



The similarity of the metapterygium and propterygium to the 

 elements of the forearm of Amphibia has struck several observers and 

 has been represented by Emery who concludes that the propterygium 

 represents the radius and the metapterygium the ulna. However direct 

 comparison appears to show that the interpretation must be reversed 

 and that the ulna is really preaxial or propterygium and the radius 

 is the metapterygium. The mesopterygium forms probably the inter- 

 medium and centralia and the chief foramen in the ossified part repre- 

 sents the intercarpal foramen. The metacarpals and phalanges may 

 have been derived in the Amphibia by a reduction of the rays of 

 Polypterus. Possibly however Polypterus may now possess more fin 

 rays than its ancestors. Evidences of the division of rays are fairly 

 frequent. It is even possible that the nerve supply from 5 spinal 

 nerves may indicate the primitive condition of 5 rays but I would 

 not wish to lay great stress on this point without further comparative 

 and embryological evidence. 



From the above it is seen that there is great probability that 

 the Pentadactyle hand is derived from a Crossoptery- 

 gian fin. One has however to account for the humerus. Previous- 

 ly I put forward a theory which I am about to repeat though I do 

 80 now with greater caution because Prof. Wiedersheim informs me 

 that it does not agree with certain embryological facts. 



