128 Bulletin American Museum of Natural History. [Vol. XXVII, 



The tympanic annulus. 



The foregoing considerations in regard to the membrana propria may 

 have some bearing on the origin of the tympanic annulus, which generally 

 first ossifies in the periphery of the membrana propria. The suggestion is, 

 that this is a secondary ossification and that the whole bone is a neomorph, 

 just as is the inner part of the tympanic or ento-tympanic (van Kampen, 

 1905, p. 705). It might be fjuestioned whether the supposed cartilaginous 

 " Anlage " of the tympanic ring which Parker (1886, pi. v) figures in the 

 embrvonic Tatusia may not be homologous with the "annulus fibro-carti- 

 laginous" of human anatomy, which is merely the thickened periphery 

 of the tympanic membrane (Cunningham, 1902, p. 706). Van Kampen 

 however (1905, p. 708), regards the tympanic bone as a transformed dermal 

 jaw element, the surangulare, while Broom homologizes the mammalian 

 tympanic with an element of the same name in the mammal-like reptiles 

 (p. 121). 



In adult Monotremes and Marsuj)ials {cf. Doran's figures, 1879) the 

 tympanic ring is closely connected with the large anterior process of the 

 malleus, which process also is of dermal origin (see p. 132). The position of 

 the tympanic ring in Monotremes and primitive Marsupials and Placentals 

 is obli([ue rather than vertical, possibly because the tympanic cavity arises 

 below and internally to the ossicular chain; and in Cynodonts (p. 127 and 

 Fig. 1, B) the tympanic membrane probably lay on a plane below the 

 quadrate and articular. 



All the above mentioned facts must be taken into account in the final 

 solution of the question whether it is a transformed jaw element, or whether 

 the tympanic bone is a neomorph in mammals, analogous in that respect to 

 the cartilaginous ear conch and to the entotympanic, and developed in 

 connection with the membrana propria or, finally, whether it is homologous 

 with the "tympanic" of Therapsids. 



Gadow's theory (1901) that the tym}>anic bone is derived from the 

 quadrate rests chiefly upon the follo\^'ing considerations: (1) In reptiles 

 the posterior border of the quadrate serves for the attachment of the tympanic 

 membrane, and in the Crocodile the quadrate and the whole auditory chain 

 suggests the mammalian conditions {cf. Figs. 5,^1, p. 134). (2) In the 

 fcetal Orycteropvs the tympanic is a semi-ring with three arms, in a measure 

 recalling the quadrate of lizards and likewise articulating by its upper 

 (proximal) end with the squamosal. (3) The knob of this tympanic 

 "almost touches" a certain process on the' mandible, which process is 

 supposed to be the homologue of the secondarily inverted angle of the mar- 

 supials. (4) This "vanishing mandibulo-tympanic joint" lies immediately 



