38o ZOOLOGY FOR MEDICAL STUDENTS chap. 



was really primitive, and common to the ancestors of all three groups, 

 or Qi) that it had been evolved independently in the three groups. But 

 we cannot suppose the latter would have happened unless this form of 

 fin were a pecuharly efficient one — in which case it is conceivable that 

 limbs of a different type might have independently become gradually 

 moulded into this form in the different groups — and we know that it 

 is not an efficient type of fin, both from what can be observed in the 

 living Ceratodus and because we know that in the couise of ages it has 

 become completely supplanted in the more successful groups of fishes 

 — the Elasmobranchs and the Teleostomes — by the other type of fin. 

 (3) There is no great difficulty in interpreting the other types of 

 Kmb skeleton on the hypothesis that they have been evolved out of a 

 biserial archipterygium. Thus in the ancient sharks of the family 

 Pleuracanthidae (Fig. 164, B) the pectoral fins had become laid back 

 alongside the body and, in correlation with this, the lateral rays on the 

 side next the body had disappeared except a few towards the tip. At 

 the same time the axis had become stouter and its component blocks of 

 cartilage larger and fewer. The condition in a young Dogfish (Fig. 164, 

 C and D) is readily correlated with that of Pleuracanihus — the axis now 

 completely embedded in the body-wall, the inner set of rays completely 

 gone except for a few vestiges near the tip in the young embryo, the 

 outer set of rays on the other hand much enlarged and forming the whole 

 support of the projecting part of the fin. Even the fin-skeleton of 

 Polypterus (Fig. 164, G) is readily derived from the archipterygium 

 when the condition in the young larva is taken into account (Fig. 164, F) 

 for it is very easy to read into the continuous plate of cartilage of this 

 stage with its radiating slits, the plan of the 'archipterygium modified 

 in a similar way to that just indicated for the sharks. 



Taking these various points into consideration it is impossible to 

 avoid the conclusion that the fin-skeleton of Ceratodus is really extra- 

 ordinarily archaic and that it is in fact a persistence of the ancestral 

 type from which other existing types of fin-skeleton have been 

 evolved. 



In the other two existing Lung - fishes, in correlation with the 

 narrowing of the limb, the lateral rays of the skeleton have become 

 reduced (Proiopterus) or have disappeared completely (Lepidosiren). 



A considerable amount of reinforcement of the original cartilaginous 

 skeleton by the formation of bone takes place in the Dipnoi. Neural 

 and haemal arches, limb girdles and cranium become in great part 

 ensheathed in, or replaced by, bone. The jaw skeleton calls for special 

 notice. The skeleton of the hyoid arch does not here, as it does in the 



