THE PECTORAL LIMB 



tissue of the flipper while its metarcarpus and all carpal sign of its 

 original situation had disappeared without leaving the faintest trace. In 

 the second place supernumerary digits, of greater or lesser completeness, 

 not infrequently occur upon the manus of divers sorts of mammals, 

 even to the point where these are perfectly functional, and this does not 

 necessitate the implication that derivation was from an ancestor which 

 normally had six or seven digits. 



In the Cetacea the rate of ossification of the digital centers is very 

 slow and one seldom encounters a specimen of sufficient age to show 

 the bony parts closely approximated each to its neighbor. More fre- 

 quently in the articulated but otherwise cleaned manus there is a con- 

 siderable hiatus between each element, this space being occupied by 

 cartilage that has shrunk as it dried. Each phalanx is flattened, truncated 

 at the ends, and somewhat constricted in the middle. The proximal bone 

 (metacarpus) of each digit is the longest and most robust, and there 

 is a progressive decrease in size distad. Almost invariably these ossicles 

 are longer than broad, but Flower and Lydekker (1891) stated that ex- 

 cept for the proximal phalanges of digits two and three, the reverse is 

 the case in Orcella. The shorter digits may have only two or three 

 phalanges, but the longer ones are furnished with a greater number, 

 there occasionally being as many as eleven (at least) elements in the 

 adult, and a considerably greater number may occur in the young (Glo- 

 biocephala, Phocaena) . This is the condition known as hyperphalangy, 

 which occurs in no mammal other than the Cetacea, although it is charac- 

 teristic of certain of the large, extinct, aquatic reptiles. Before con- 

 sidering the possible explanations for the manner in which this has been 

 brought about it will be well to examine the conditions now obtaining 

 in cetacean digits. 



Flower (1876) has stated that in the Odontoceti the phalanges are 

 often connected together by imperfect synovial joints. So far as I know 

 no other investigator has independently made this claim and although 

 it cannot be denied unqualifiedly, I would consider it as extremely un- 

 likely that this is ever the case. There is not the faintest sign of a 

 synovial cavity in the young odontocetes which I have had the oppor- 

 tunity of examining and it is not likely that such would develop only 

 in adult life. In a cetacean fetus of say one third term size, it is seen 

 that each digit is composed of normal hyaline cartilage with a small 

 center of ossification at its middle. Investing each cartilage is what 

 appears to be perichondrium, and this covers the ends of the elements 

 as well. There is not the least sign of an irregularity, cavity, or articular 



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