93 



NOTE ON THE HAND SKELETON OF SOME CETACEAN 



FOETUSES. 



By Stanley T. Bukfield, B.A. Cantab., 

 Lecturer in Zoology, Liverpool University. 



The Cetacean fore-limb has formed the subject of a number 

 of researches. Taking as a starting point the almost 

 universally accepted assumption that the Cetacea have been 

 evolved from land-living mammals, the changes which have 

 been brought about in the derivation of the balancing and 

 guiding flipper* from the typical pentadactyl arm and hand of 

 the land-livers, form a fascinating study. 



The main lines of the adaptation of the pentadactyl 

 fore-limb to the aquatic habit may perhaps be restated. The 

 humerus is short but freely articulated at its joint with the 

 shoulder girdle. The flipper as a whole moves about this 

 joint, and all the remaining joints in the fore-limb are imperfect. 

 The radius and ulna are flattened, and these bones, together 

 with the wrist and digits, are bound together to form a single 

 smooth-surfaced " fin " or flipper. In the production of this 

 fin-like fore-limb the digits are often spread out, and the 

 number of phalanges has been increased (Hyperphalangy). 

 This latter phenomenon is shown in a still more marked degree 

 by the Ichthyosauria, the adaptation in this case having been 

 likewise brought about, it is believed, in the derivation of a fin 

 or flipper from a typical pentadactyl fore-limb of some land- 

 living reptiles. This similar adaptation in two such widely 

 separated groups as the Cetacea and Ichthyosauria exhibits 



* It is probable that the flipper was, in the first instance, a swimming 

 paddle relatively larger than is now seen in the living Cetacea. With the 

 taking over of the propulsion by the tail, the flipper became relatively smaller 

 and retained only the function of guiding and balancing as in the pectoral fin 

 of fish. 



