1010.] The Tympanic A^imdus. 129 



behiiul the c-artila<>-iii()us pr()l()n<i-ati()n of the Meckelian cartilage /. c, in the 

 approximate })o.sition of the (luath'ate-artieular joint of reptiles. From these 

 ])arallel relations of (|ua(lrate and tvm})anic to the mandible, and relying on 

 the other considerations already enumerated, Dr. Gadow concludes that the 

 tympanic is homologous with a ])art at least of the (piadrate, and he holds 

 that the inner or posterior angle of mammalian jaws is also a reminiscence 

 of this former mandibulo-tympanic connection. This argument appears to 

 be open to several objections as follows: 



(1) It seems strange that the Crocodile, a highly specialized predatory 

 reptile of aquatic habits and belonging to an order which is very remote 

 from the ancestry of the mammals, should be supposed to have retained in 

 far greater degree than does Sphenoden and other reptiles the j)rimitive con- 

 .ditions which furnish the key to the homologies of the mammalian ossicula. 



(2) In the other direction Orycferopus, it is true, is in many respects a 

 primitive Placental Mammal, but the structural gap between all Placentals 

 and all modern reptiles is so almost immeasurably great that a sporadic 

 resemblance between the relations of the tympanic in the foetal Oryctcropus 

 and of the ipiadrate in the Crocodile seems to be a highly unreliable item 

 of evidence in support of so far reaching a theory. Of very different charac- 

 ter is the evidence (reviewed below) for the homology of the quadrate and 

 articular with the incus and malleus respectively, since this is founded on 

 several independent lines of comparison between all reptiles on the one hand 

 and all embryonic mammals on the other. And if to this it be replied that 

 the force of the comparison between the Crocodile and the foetal Orycteropus 

 rests not in the separate items themselves but in the totality of the resem- 

 blance, the rejoinder is that the validity of the homology between the (juad- 

 rate and the tympanic rests upon the validity of the homology between the 

 extracolumella and the malleo-incudal mass, a homology which is shown 

 below to encounter serious objections. 



(3) While the posterior border of the quadrate serves to support the 

 tympanic membrane in the Crocodile and other recent reptiles this could 

 not have been the case (at least to the same extent) in the Cynodonts, whose 

 claims to relationship with the mammals are certainly at least worth con- 

 sidering. In the Cynodonts the reduced condition of the quadrate and the 

 position of the supposed osseous auricular meatus indicate that the tym- 

 panic membrane was possibly stretched in part upon the descending flange 

 of the squamosal (Fig. 1, B, ty. m.). We conclude therefore that Dr. 

 Gadow's case for the derivation of the tympanic from the c{uadrate has not 

 been made out. 



