GENERAL CHARACTERS 9 



developed into mammals, in which the mandible articulates 

 directly with the squamosal, and consists of only a single 

 element on each side, corresponding to the dentary bone in 

 the reptilian lower jaw. According to one of the latest author- 

 ities ' on this intricate subject, it seems probable that the 

 reptilian quadrate has been taken up into the internal ear of 

 mammals to form the incus, while the articular, or hindmost 

 bone, of the lower jaw of the former has likewise been incor- 

 porated in the same region of the ear, where it forms the 

 malleus. It is added that a meniscus of cartilage found between 

 the condyle of the lower jaw and the squamosal in most mam- 

 mals is a new element, unrepresented in reptiles. 



Another interpretation of the fate of the quadrate is, how- 

 ever, suggested by Dr. R. Broom, 1 ' who has paid particular at- 

 tention to the relationship existing between the theriodont 

 anomodonts and mammals. After pointing out that, according 

 to his interpretation, the quadrate in the theriodonts is reduced 

 to an exceedingly small bone affixed to the extremity of the 

 squamosal, the author suggests that the reptilian quadrate is 

 represented by the aforesaid meniscus, or interarticular cartilage, 

 in mammals. " On the other hand," he adds, " it is quite pos- 

 sible that the quadrate is entirely absent in all mammals ; yet 

 the presence of a cartilaginous element in a situation exactly 

 corresponding to that of the quadrate in theriodonts seems 

 strongly to favour the view that in the meniscus we have the 

 modified equivalent of the reptilian quadrate." 



Whatever may be the ultimate verdict on this difficult 

 question, it is quite evident that among the theriodont anomo- 

 donts there must have existed forms in which the quadrate 

 bone had more or less completely lost its articular function, 

 and in which the two branches of the lower jaw had well-nigh 

 discarded all their separate elements situated posteriorly to 

 the dentary. 



The mode in which this transition from the reptilian to the 

 mammalian type has been effected is practically demonstrated 

 by specimens figured in Dr. Broom's paper. The quadrate, ac- 

 cording to the author's interpretation, forms a small knob to the 

 descending process of the squamosal in the theriodonts, and has 



1 Dr. Kjellburg, in Gegenbaiw's Morphologisckes Jnhrhucli for 1904. 

 "Proceedings Zool. Soc. London, 1904, vol. i., p. 490. 



