THE CnOXDROCRAXIUM OF LEPIDOSTEUS. 263 



recessus epitympanicus, this recess, as thus described, thus lying 

 wholly on the external surface of the otic capsule, and not partly 

 on the lateral wall of the cavum supracoehleare. The upper 

 one of the two enclosing ridges forms the I'oof of the recessus 

 epitympanicus, and corresponds exactly to the roof of that recess 

 of Voit's descriptions of the rabbit, and it is said that it is this 

 roof alone that forms, in human anatomy, the tegmen tympani. 

 The tegmen tympani of human embryos, as thus defined, is thus 

 a ridge Avhich forms a part of the external, instead of the internal, 

 wall of the recessus epitympanicus, and hence is in no way 

 comparable to the tegmen tympani of the adult Man, where it is 

 a thin plate of bone which lies between the tympanic and cerebral 

 cavities and separates them one from the other. In Mammals 

 other than Man,'"the recessus epitympanicus is said to be less tall, 

 and as, in consequence of this, the roof of the recess and the 

 crista facialis are not distinctly separated from each other, they 

 are both considered to be included in the tegmen ; the tegmen 

 of these Mammals, as thus defined, thus corresponding to the 

 tegmen of the rabbit as described by Yoit, and forming 

 the mesial wall and part of the external wall of the recessus 

 epitympanicus. The crista facialis alone is said by Yan 

 Kampen to probably be the homologue of the crista parotica of 

 Lacerta, and Gaupp ('08, p. 687) accepts this as correct. Gaupp 

 also further says that the tegmen tympani is a new formation 

 which first appears in Mammals, but that it is wanting in Echidna ; 

 and yet he shows, in one of his figures (l. c. fig. 44, p. 628), a 

 portion of the crista parotica which has, to the incus, exactly the 

 relations of that part of the tegmen tympani of both Voit's and 

 Van Kampen's descriptions that forms the roof of the recessus 

 epitypanicus. Van Kampen, in the figure of a new-born Oris, 

 shows the tegmen tympani as a right-angled ridge the anterior 

 portion of which forms the roof of the fossa muscularis major 

 and ends in the transverse plane of the hiatus facialis. The 

 anterior portion of the tegmen is said by Van Kampen to be 

 very variable, and to often be wholly wanting, and in Man it is 

 said, to arise, not as a ridge, but as an anteriorly projecting 

 process, the processus perioticus superior Gradenigos. The 

 process lies, as shown in the figure given {I. c. fig. 3, p. 342), 

 definitely anterior to the recessus epitympanicus, and anterior 

 also to the column of cartilage that forms the anterior boundary 

 of the foramen faciale externum s. secundarium ; and the 

 ossification of this process and the layer of connective tissue 

 that separates it from the pars cochlearis of the otic capsule is 

 said to form the tegmen tympani of the adult. The mesial wall 

 and the roof of the recessus epitympanicus of embryos thus, 

 neither of them, enters into the tegmen tympani of the adult. 



There is thus some want of accord and precision in the 

 descriptions of the tegmen tympani by these authors, and this 

 has in a measure been perpetuated in later descriptions. Fawcett, 

 for example, says ('17, pp. 319-321) that the tegmen tympani of 



