VIIL] HOMOLOGIES. 201 



identity. But the Amphibia and Plesiosauria, though 

 not themselves primitive vertebrate types, may be thought 

 by some to have derived their limb- structure by direct 

 descent from such. Tortoises, however, must be admitted 

 to" be not only highly differentiated organisms, but to be 

 far indeed removed from primeval vertebrate structure. 



^"^^^m^^m^m^^^ 



SKELETON OF A PLESIOSAURUS. 



Yet certain tortoises^ (notably Chclydra Tcmmincldi) 

 exhibit such a remarkable uniformity in fore and hind 

 li.mb-structure (extending even up to the proximal ends 

 of the humerus and femur) that it is impossible to doubt 

 its independent development in these forms. 



Again in the Potto (Perodicticus) there is an extra bone 

 in the foot, situate in the transverse ligament enclosing 

 the flexor tendons. It is noteworthy that in the hand of 

 the same animal a serially homologous structure should 

 also be developed.^ In the allied form called the Slow 

 Lemur (Xycticebus) we have certain arrangements of the 

 muscles and tendons of the hand which reproduce in great 

 measure those of the foot and vice versd.^ And in the 

 Hyrax another myological resemblance appears.* It is, 



1 An excellent specimen displaying this resemblance is preserved in the 

 Museum of the Royal College of Surgeons. 



2 piiii. Trans. 1867, p. 353. 



3 Proc. Zool. Soc. 1865, p. 255. ^ n^ij. p. 351. 



