VIII.] HOMOLOGIES. 193 



If they have descended from such primitive forms as 

 Prof. Gegenbaur imagines, then they have built up a sec- 

 ondary serial homology a repetition of similar modifica- 

 tions fully as remarkable as if it were primary. The Ple- 

 siosauria those extinct marine reptiles of the Secondary 

 period, with long necks, small heads, and paddle-like limbs 

 are of yet higher organization than are the efts and other 

 Amphibia. Nevertheless they present us with a similarity 

 of structure between the fore and hind limb, which is so 

 great as almost to be 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, how- 

 ever, must be admitted to be not only highly differentiated 

 organisms, but to be far indeed removed from primeval 

 vertebrate structure. Yet certain tortoises aa (notably Che- 

 lydra Temminckii) exhibit such a remarkable uniform 

 ity in fore and hind limb 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, situated in the transverse ligament enclos- 

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

 of the same animal a serially homologous structure should 

 also be developed. 23 In the allied form called the slow 

 lemur (Nycticebus) we have certain arrangements of the 

 muscles and tendons of the hand which reproduce in great 

 measure those of the foot, and vice versa. And in the 

 Hyrax another myological resemblance appears. 26 It is, 



22 An excellent specimen displaying this resemblance is preserved in 

 the Museum of the Royal College of Surgeons. 



23 Phil. Trans., 1867, p. 353. 



24 Proc. Zool. Soc., 1865, p. 255. 



25 Ibid., p. 351. 



