AQUATIC MAMMALS 



the structures are entirely homologous. It seems odd that the turtles 

 have apparently been unable to attain the condition of hyperphalangy, 

 although of course it is entirely possible that the stimuli which they 

 have encountered were of a different sort. Be that as it may, in spite 

 of the fact that the manus of the most specialized turtles have the pha- 

 langes very much elongated, no additional elements have been acquired. 

 Presumably their flippers are entirely satisfactory as swimming organs 

 except for the fact that the length of the phalanges renders the bones 

 more liable to breakage, and signs of the previous fracture of the manus 

 is frequently to be seen in skeletons. In such turtles as the leatherback 

 the fact that the first digit is not much the heaviest, in spite of the flipper 

 being used as an oar, is without great significance, for it is not elongated 

 like the middle three. 



(3) Perhaps the majority of present-day cetologists incline to the 

 theory of hyperphalangy by intercalary syndesmoses. Howes and Davies 

 advanced this thesis as applying to the Cetacea after having studied the 

 small accessory phalangeal element in several amphibians. Their con- 

 tention was that the interosseous spaces are filled with fibrous tissue which 

 takes the place of synovial joints. This structure might appear to be 

 a derivative of the phalangeal investing tunic, but rather is it a differen- 

 tiation of the mass from which the phalanges themselves are derived, 

 although intimately related to the sheath; so that the phalanges and 

 syndesmoses are, together with the investing sheath, differentiations of 

 a continuous common blastema. Thickenings of the fibrous interosseous 

 tissue might then become phalangeal elements. Thus is indicated a pos- 

 sible intercalary origin, from articular syndesmoses, of the supernumerary 

 phalanges of the Cetacea. This may have been associated with loss of 

 the ungues in a manner similar to that in which elongation, by regular 

 segmentation, of the cartilaginous rays in the paired fins of the Batoidei 

 (skates and rays) would appear to have been connected with the dis- 

 appearance of the horny fin rays. 



I have no criticism to make of the above facts and theories in so far 

 as they concern amphibia, but they should not have been applied to 

 the Cetacea. In the first place, as these intercalary bones are derived 

 from nbro-cartilage they are in effect nothing but sesamoids and al- 

 though occasionally these may reach a considerable size, there is no 

 reason for believing that in mammals they can ever develop to the exact 

 size and proportion of a phalanx derived from hyaline cartilage, nor 

 that such could ever truly become interpolated as a part of the digital 

 series. Even if they could do so in amphibians it is no proof that this 



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