On the Fate of the Quadrate in Mammals. 409 



whicli they had to contend against, we may judge an indi- 

 vidual by its external appearance in the last resort only ; we 

 determine its systematic position far rather from its internal 

 organization, as displayed to us by means of anatomy and 

 histology, and above all from its embryology. 



The latter lead us to the conclusion that Nemertines have 

 probably been derived from Turbellarian-like forms, but that 

 after following a line of development over which the Annelids 

 had already passed, they diverged from it again in a direction 

 of tlieir own. 



Gottingen, Sept. 1889. 



XLVIII. — On the Fate of the Quadrate in Mammals. 

 By E. BiioOM, M.B., CM., B.Sc. 



One of the most troublesome points in the study of the 

 descent of the Mammalia is the explanation of the changes 

 which have taken place in the structure of the lower jaw and 

 in its mode of articulation with the skull. In Amphibians 

 and Reptiles the lower jaw is invariably made up of a number 

 of pieces and articulates with the skull by means of the 

 quadrate. In Mammals the jaw is apparently a single bone 

 articulating with the squamosal. What we have therefore to 

 explain is. What has become of the quadrate and how has 

 the jaw become simplified ? In the present paper I shall 

 only deal with the fate of the quadrate. 



Hitherto the majority of comparative anatomists, chiefly 

 from the study of the early condition of the visceral arches, 

 have agreed in finding the homologue of the quadrate in one 

 or other of the auditory ossicles. Gegenbaur, Kolliker, 

 Wiedersheim, and Keichert find its representative in the incus, 

 while Huxley looks upon the malleus as its equivalent. 

 Parker, who has done more than anyone else to elucidate the 

 development of the skull, after for many years holding the 

 same view as Huxley, ultimately came to regard the incus as 

 the Mammalian quadrate. 



That the quadrate of the Amphibian or Keptilian ancestors 

 of the Mammals should gradually move back from the arti- 

 culation of the jaw and degenerate into one of the auditory 

 ossicles is improbable ; and there is little doubt but that the 

 view has been founded on a misinterpretation of the morpho- 



