264 MR. E. p. ALLIS ON THE OTIC REGION OP 



Microtus is a curved shell-like cartilage which projects forward 

 from the base of the triangular lateral surface of the otic capsule, 

 "over the incus and malleus cartilages"; that the reeessus 

 epitympanicus has been formed by the hollowing out of the 

 under surface and root of the anterior prolongation of the 

 tegmen tympani ; and that the posterior portion of the latter 

 recess has been deepened to form a fossa incudis which lies on 

 the " upper aspect" of the crista parotica. The tegmen tympani 

 of this mammal, as thus described, would thus seem to be 

 nothing more than the ridge that forms, in Yoit's descriptions of 

 the rabbit, the roof of the fovea epitympanica ; and it would 

 seem as if it could not be an anterior prolongation of the crista 

 parotica, for its posterior portion, which forms the dorsal 

 boundary of the fossa incudis, must lie either on the external 

 surface of the crista parotica, or on the external surface of the 

 otic capsule immediately dorsal to that crista. In Erinaceus 

 the tegmen tympani, as described by Pawcett ('18), also forms 

 the roof of the recessus epitympanicus, but it would here seem to 

 be a direct anterior prolongation of the crista parotica. Terry 

 ('17) does not consider that part of the ridge of these descriptions 

 that forms the'roof of the fossa incudis to be a part of the tegmen 

 tympani, for he says {I. c. p. 300) that the tegmen tymjDani is 

 liot present in a 23-1 mm. embryo of the cat, and yet he shows, 

 in his figure 3, a well-developed fossa incudis which is bounded 

 dorsally by a strongly marked but not projecting ridge which is 

 evidently the homologue of the posterior portion of the roof of 

 the fovea epitympanica of Yoit's, Van Kampen's, and Fawcett's 

 descriptions. The mesial wall of the fossa incudis is apparently, 

 and correctly, not considered by either Fav.cett or Terry to form 

 part of the tegmen tympani. 



Comparing- these conditions in Mammals with those in Fishes, 

 it is evident that that part of the tegmen tympani of Voit's 

 descriptions of the rabbit that forms the lateral wall of the cavum 

 supracoehleare, together with its anterior procartilaginous pro- 

 longation, is the post-trigeminus portion of the lateral wall of the 

 pars ganglionaris of the trigemino-facialis chamber of Fishes. 

 That so-called part of the tegmen that forms the roof of the 

 fovea epitympanica must then be the .sphenotic portion of the 

 spheno-pterotic ridge of Fishes. The incus, which lies immediately 

 ventral to this roof, then has the relations to it that it normally 

 should, if, as I have latelyendeavoured to sho\v^(Allis, in press), it 

 is the homologue of the otic process of the quadrate of Ileptanckus, 

 of the corresponding part of the quadrate of the Amphibia and 

 Reptilia, and of the lateral wall of the pars jugidaris of the 

 trigemino-facialis chamber of Fishes. The crista facialis certainly 

 corresponds to the anterior portion of the opisthotic ridge of 

 Poli/j:)terus, and the crista parotica is usually shown as a direct 

 posterior continuation of this crista, and not of the roof of the 

 fovea epitympanica. The crista parotica would thus seem to cor- 

 respond to tlie posterior portion of the opisthotic ridge of Fishes, 



