1910.] Summary of the Relations of Cynodontia to Mammalia. 143 



secondarv and to have resulted from the proxhiaity of these elements during 

 the course of development. Broom, however, has cited supposedly analo- 

 gous cases where structures which are known to be distinct morphologically 

 become connected in ontogeny and arise from a continuous cartilage. 



(2) The fate of the cpiadrate is still unsettled. According to Gadow 

 a ])art of it at least has become transformed into the mammalian tympanic 

 bone, but this view appears to have extremely little evidence in its favor. 

 According to Broom the ([uadrate has been reduced into the interarticular 

 disc or meniscus. Broom interprets the conditions in the Cynodontia as 

 pointing in this direction, but the ontogeny of the mammals has so far not 

 lent any direct support to this view, but has been interpreted by Kjellberg 

 as indicating that the meniscus is a neomorph, derived from the fibres of the 

 pterygoid muscles. 



(3) The general theory in question assumes that the crocodile is more 

 primitive than Sphenodon in the condition of its auditory ossicles, a fact 

 which appears contrary to what might have been expected. 



(4) The suprastapedial, which Broom homologizes with the incus, is an 

 inconstant element in the Reptiles, which show a wide range of variability 

 in the appendages of the columella (Versluys) and therefore the supra- 

 stapedial is hardly free from the suspicion of being a neomorph. 



(5) The theory under consideration must assume that the chorda 

 tympani, which lies above the ossicular chain in the reptiles has passed 

 through it and come out on the under side (Broom), since it appears below 

 the chain in mammals. Such migrations of nerves through bones are of 

 course not rare (Broom) and the variable position of the carotid foramen 

 in a single family, the Viverridse, appears to be a case in point. 



Whichever of these two theories shall finally be adopted will apparently 

 be entirely reconcilable with the view that the mammals have been derived, 

 not from unknown Devonian amphibia, not from streptostylic reptiles allied 

 to the lizards, but from as yet undiscovered Synapsid reptiles of the order 

 Cvnodontia. 



