AQUATIC MAMMALS 



There would also seem to be another character which the manus of the 

 aquatic mammal may be expected ultimately to adopt. In most terrestrial 

 mammals the bones of the metacarpus and digits consist of a shaft and but 

 a single epiphysis. The latter is situated upon the the distal end of the 

 shaft in the four lateral metacarpals, but at the proximal end of the first 

 metacarpal and all the phalanges. Apparently there is a tendency in 

 aquatic mamals for all of these bones to develop an epiphysis at each 

 end. Kiikenthal (1891) appeared to think that this is attributable to 

 the retarded rate of ossification in the elements of the manus. Double 

 epiphyses are well differentiated in distal digital bones of at least 

 most cetaceans, and Kiikenthal stated that this character is advanced in 

 sirenians, and among pinnipeds in Mirounga, Hydrurga ( = Stenorhyn- 

 chus), Otaria, with indications in Odobenus, and in the pes only of 

 Cystophora. But Flower (1876) said that they are double in the manus 

 only of Mirounga, while Weber (1886) stated that in the pinniped 

 manus the epiphyses are normal, being double only in the proximal 

 elements of the pes. Kiikenthal also said that there are indications of 

 double epiphyses in Ormthorhynchus, Hydromys, Hydrochoerus and 

 Castor. 



All this is as may be. To establish the incontrovertible presence of 

 double epiphyses is not always easy without extremely careful investi- 

 gation, and without taking any especial pains in preparation I have been 

 able to satisfy myself that this situation obtained only in the case of 

 sundry Cetacea and a very few pinnipeds of the proper age to exhibit 

 the character to good advantage. I might point out two likelihoods, 

 however ; that it is extremely easy to interpret cartilaginous irregularities 

 at the end of the phalanges as early stages of epiphyseal ossification, 

 andy that if one searched for this condition among strictly terrestrial 

 mammals with as much zeal as it has been sought in partially aquatic 

 forms he might very well have equal success. 



It is almost an invariable rule that when a mammal first takes to the 

 water the use of its anterior limbs as natatory organs will be merely 

 incidental. The probabilities are that these members will be used with 

 the hind limbs in swimming "dog-fashion" but the important function 

 of all four legs is for terrestrial progression. As the hind feet are 

 ordinarily larger than the anterior, these will have a tendency to increase 

 in proportional importance for propulsion. In most cases I believe 

 that the aquatic development of the manus will be extremely slow, for 

 it has not sufficient area to be of decided import either in steering or 

 swimming. But bodily conformation may introduce disturbing ele- 



[242] 



