OF THE MAMMALIAN OSSICULA AUDITUS. 493 



3. That a definite incus has long been admited to exist, articulated, as above noticed, 

 with the malleus, and ankylosed in the Echidna, and generally in the adult Ornitho- 

 rhynchus. The method of ankylosis is quite different from that seen in certain Rodents. 



4. That as a processus longus of the incus exists, and hence in all probability is the 

 proximal end of the second postoral arch, it follows that the column of the stapes of 

 these animals (and of all other mammals where it is of similar shape) represents the two 

 crura of the stapes of higher mammalia, but only a part of the column of the Sauropsidan 

 columella, which is generally admitted to be formed partly from the " mediostapedial " 

 element of the hyoid arch. But a further careful study of the embryo of these animals 

 is necessary to settle this point otherwise than by analogy. 



5. That the malleus and incus of Omithorhynchus differ from those of Echidna in 

 matters of detail, but in no essential features which might ally either animal more or 

 less than the other to higher mammals or to the Sam*opsida, excepting, some might say, 

 the semimembranous consistence of the malleus of the Duck-bill. 



DESCRIPTION OP THE PLATES. 

 Plate LVIII. 



Throughout the Plates the left malleus and incus are represented as viewed from the inner side, that 

 position being the most convenient for displaying the homologous elements. The " natural position," 

 i. e. that in which the manubrium of the malleus is turned towards the observer, is objectionable, because 

 foreshortening of the ossicles interferes with a ready comparison of their various processes and relative 

 dimensions as seen in different animals. The ossicula are enlarged 3 times natural size, unless otherwise 

 specified. The letters used in the various figures bear the following signification : — as, articular surface 

 of malleus ; n, neck ; pb, processus brevis ; pg, processus gracilis ; a, a*, upper facet, and b, b*, lower 

 facet of malleus ; sr, sigmoid ridge of malleus ; e?nn, external surface of manubrium ; pi, processus 

 longus ; c, concavity head of malleus ; eas, extra articulating surface on malleus and incus ; gr, groove 

 on incus; aw, angular projection on neck of malleus; oa, orbicular apophysis ; mall, malleus ; Im, fig. 22, 

 PL LX., in error for pm ; pm, processus rnuscularis ; hm, head of malleus (fig. 1, PL LXIIL, lettered 

 wrongly for ran, see footnote p. 458) ; pg, processus gracilis ; I, lamina ; phm, process head of malleus ; 

 acpm, accessory processus rnuscularis ; inn, manubrium ; pet, petrous bone ; tb, tympanic bone ; shm, 

 scale head of malleus ; ms, medio-stapedial element. 



Figs. 1-17. Left ossicula : 1,2, Homo* ; 3, Troglodytes gorilla ; 4, Troglodytes niger ; 5, Simla satyrus ; 

 6, Hylobates syndactylus ; 7, Semnopithecus entellus; 8, Colobus satanas ; 9, Cercopithecus cephus • 

 10, Macacus erythraus ; 11, Cynopithecus niger ; 12, Cynocephalus anubis ; 13, Ateles melanochir > 

 14, Mycetes ursinus ; 15, Cebus ; 16, Nyctipitkecus trivirgatus ; 17, Saimaris sciurea. 



Fig. 18. Left malleus and incus : Pithecia satanas. 



Figs. 19-22. Left ossicula: 19, Hapale jacchus ; 20, Midas cedipus ; 21, Lemur albifrons; 22, Galago 

 crassicaudata. 



* The remaining figures of the left ossicula show the malleus placed as in fig. 2, and the incus as in fig. 1. I had 

 intended that the human malleus and incus should he placed together in the desired position in the same group — 

 namely, fig. 2 ; hut by an oversight the object was not reversed on being drawn on stone. 



SECOND SERIES. — ZOOLOGY, VOL. T. 3 V 



