COMMON FINBACK WHALE. 



191 



surfaces. The radius and ulna are flattened and exceed the humerus in length. The elbow 

 is well devekiped as a backward prolongation of the ulna extending up a short distance along 

 the posterior side of the humerus. The bones of the wrist are somewhat poorly developed 

 as ossifications in the great mass of cartilage corresponding externally to the base of the flipper 

 or pectoral limb. In an adult whale there are six of these bones arranged in two rows: four 

 in the proximal and two in the distal row. Those of the first row are probably homologous 

 with the radiale, intermedium, and ulnare of the typical vertebrate carpus with a large pisi- 

 form at the external side; possibly, however, the first represents a fused prepollex and radiale, 

 as there is some evidence of two centers of ossification in this bone. The homology of the 

 two small bones of the distal row is of great interest. In Balaenoptera as in Megaptera, there 

 are but four fingers in the hand, and it has been generally assumed that, as is usual in cases 

 of digit reduction, it is the thumb that has become lost. Kiikenthal (1893), however, made 

 the remarkable discovery that it is probably the third digit instead. For in at least two 

 embryo Finbacks he found loosely imbedded in the tissue between the second and third fingers, 



Text-figs. 5, 6, 7. — Shoulder blades of wlKdebonc wliides (from True, l'J()4). 

 .5. — -North Atlantic Right Wha'e (Eubalacna glaciaKs). 

 6. — Common P^inback What' {Bataennptera physalus). 

 7. — Humpback Whale {Megaptera nodosa). 



several vestigial phalanges which, he urges, ])robably represent the true third or middle finger. 

 That this contention is correct, is additionally shown by the fact that the median nerve of the 

 arm gives off' hvo branches to the space between the present second and third fingers. In 

 five-fingered animals there is one branch to each interspace, hence the presence of two branches 

 in this case points to the former existence of another digit here. It follows, that since only 

 the tip of this digit is still left in occasional specimens, its disappearance must have begun at 

 the base, hence the corresponding carpale 3 may be considered lost. Leboucq has described 

 a double ossification in the inner of the two distal carpalia, so it is considered by Kiikenthal 

 that this single bone represents a fusion of carpalia 1 and 2. The other existing bone is there- 

 fore either the carpale 4 or a fusion of the carpalia 4 and 5 of the primitive vertebrate hand. 

 There is, however, no positive evidence that it represents a fusion of two elements. 



