ON THE OUTLOOK OF VERTEBRATE MORPHOLOGY. 79 



appearing in 1889 and the second in 1893. an d it is divided 

 into six chapters of variable length. The first of these is 

 devoted to a full description of the skin, and, returning to 

 this in a later chapter, the author, confirming and extend- 

 ing Gray's discovery of dermal ossicles in the Delphioidea, 

 builds up an argument that the toothed whales may have 

 had mailed ancestors. He deals with the development of 

 the limb skeleton in an almost exhaustive manner ; and not 

 the least remarkable discovery is that the numerical reduc- 

 tion of the digits to four in the whalebone whales, other 

 than the Balasnida^, well known, is in Balcenoptera mus- 

 culus due to the loss of the third digit, and not to a 

 suppression from within outwards, supposed to be the rule 

 among mammalia generally — a fact incidentally the more 

 interesting and suggestive in consideration of Forbes's 

 discovery 45 that in tridactyle birds it is not always the 

 hallux that is suppressed. In dealing with the vexed 

 question of hyperphalangy, the author formulates the facts 

 observed by himself and others concerning the numerical 

 variation of the phalanges at different stages of growth, 

 and shows that in all known cases their number diminishes 

 with age — a most interesting feature, as remarkable as 

 unexpected, inasmuch as it proves those forms which, like 

 the Platanista, may in the adult state present the character- 

 istic mammalian formula 2 . 3 . 3 . 3 . 3, to be thus far the 

 most highly modified members of the order. In estimat- 

 ing the morphological value of the supernumerary pha- 

 langes, he avails himself of Leboucq's discovery 46 of the 

 nail rudiment and its apparent constancy of relationship 

 to the free end of the digit at observed stages in growth, 

 a discovery which, whatever the value of supernumerary 

 elements, proves these to be intercalary, and deals the 

 death-blow to Ryder's fascinating hypothesis 47 of their 

 origin in catilaginous rays of the eared-seal type. Kliken- 

 thal believes the "supernumerary phalanges" to be dupli- 

 cated and dismembered epiphyses, and in support of his 

 view falls back upon the discovery by Allen Thomson 48 

 and others of the occasional presence of these among the 

 more normal mammalia. He records the existence of 



