66 The Phylogeny of tlie Crural Flexors 



stress upon the suppl}- of the gastrocnemius medialis by a brancli of the 

 external saphenous nerve. I have not been able to trace the origin of 

 the nerve in the opossum, but one must conclude from Cunningham's 

 statement, 8i, that in the thylacine the nerve arises from the internal 

 popliteal. In these two forms then, the thylacine and Phalangista, two 

 different origins of .the nerve occur, one of which favors Eisler's migra- 

 tion theory while the other is opposed to it. Which is the more primi- 

 tive origin? I have not been able to find in the literature accessible to 

 me any sufficiently detailed accounts of the arrangement of the nerves 

 in other marsupials or in the monotremes, but, since there can be no 

 question as to the identity of the lacertilian muscle termed above the 

 plantaris superficialis medialis with the mammalian gastrocnemius inter- 

 nus, the origin of its nerve fibers may throw some light on the question. 

 In Scincus it is supplied by a branch from the ramus superficialis medi- 

 alis, i. e., from the more medial of the two superficial nerve trunks and 

 according to Gadow, 82, this is the usual condition in the lacertilia which 

 he studied, in Ophryoessa only does the branch come from the ramus 

 superficialis fibularis. In the crocodiles the muscle is supplied by a 

 branch from the ramus profundus and a weak branch from the super- 

 ficialis medialis, while in the alligator it receives branches from both 

 superficial nerves, that from the fibularis being the smaller. It seems, 

 therefore, that there is a considerable amount of variation in the course 

 of the nerve fibers in question, a fact which weakens an argument based 

 solely on the path followed by a group of nerve fibers in a single species 

 of mammal. 



It seems to me that the muscle in question is primarily and finally 

 a muscle of the tibial side of the crus, and that its homologue in that 

 position can be found from the nrodele amphibia to the highest mam- 

 malia. Eisler, as has already been pointed out, has failed to recognize 

 the true plantaris superficialis of the amphibia and has thus been led 

 widely astray in his attempts to homologize the amphibian and mam- 

 malian muscles. He finds the amphibian homologue of the gastroc- 

 nemius medialis in the fibulo-tarsalis (fibulo-plantaris, Eisler) and that 

 of the gastrocnemius lateralis in the plantaris profundus III minor 

 (plantaris superficialis minor, Eisler). It may be pointed out that 

 both these muscles lie beneath the plantaris profundus III (plantaris 

 superficialis major, Eisler) which Eisler identifies with the mammalian 

 plantaris. This latter muscle, however, wherever it can be certainly 

 identified, is in relation with the deeper portion of the gastrocnemius 

 lateralis and would seem to be a derivative of the deeper portion of 

 that muscle. Eisler's identifications would according^ require an in- 



