454 SCIENCE PROGRESS 



fins of fish like the eel. But the main burden of propulsion 

 was borne by the tail. In performing this function the tail 

 and a greater part of the trunk would be constantly bending 

 first to one side and then to the other. This would not in- 

 convenience the median folds, on the contrary they would help 

 to give the tail a greater purchase on the water. On the other 

 hand the paired folds would be alternately stretched and 

 crumpled, and the enclosed rays parted and crowded in a way 

 that would be a distinct disadvantage. Only in two regions 

 would this not take place : the one near the head, where of 

 course the bending of the body was at a minimum or even 

 non-existent ; the other at the point where the folds converged 

 and where, as a consequence, they would be hardly more 

 affected by the bending than would the median fins. The 

 disappearance of the folds in the region between these two 

 would naturally follow. It may be explained as being due 

 either to mechanical, selective or other influence according to 

 the predilections of the reader. In some such way as this 

 the paired lobes seen in Cladoselache may have originated. 



At first the base of attachment of the lobe thus isolated 

 must have been very broad as it is even now in the early 

 stages of development of the sturgeon.^ But the same in- 

 fluences which lead to the isolation of the two pairs of lobes 

 would lead also to a gradual narrowing of the base. At the 

 same time the outward extension of the lobe would add to 

 its efficiency as a keel. 



The skeleton of the fin in Cladoselache (fig. 12) consisted 

 of a basal portion within the body wall and a rayed portion 

 in the lobe itself. The front rays were stout and formed a 

 firm keel. The hinder rays were thin and the corresponding 

 margin of the fin extended into a membrane. It is quite 

 possible that this margin could be turned up or down slightly 

 and so could be used for guiding the fish to different levels. 

 The attachment of this margin to the body wall would however 

 limit its usefulness in this direction. It is evident that if this 

 margin were released from the body wall a double advantage 

 would be gained, since the base of the fin would become corre- 

 spondingly narrower and at the same time the hinder portion 

 of the fin would be freer to perform tilting movements. The 



' S. Mollier, " Die Entwickelung der Paarigen Flossen des Stors," Atiaio- 

 mische^ Hefte Bd., 1897, p. 16. 



