224 DUBLIN UNIYEB91TT 



pear, in Marsupialia — absence of the corpus callosum. The two princi- 

 pal commissures are the anterior and the fornix, both well developed, 

 the latter being continuous behind with the hippocampus major — itself 

 much developed in such animals as have large olfactory bulbs and 

 tracts with which it is connected. The difference constituted by the 

 existence of convolutions in the one, and none in the other, is remark- 

 able. The remaining parts do not much differ from other quadrupeds, 

 and may be understood by glancing at the accompanying drawings. 



"With respect to the organs of sense and the cerebral nerves, we may 

 commence (having already noticeed that the olfactory organs are enor- 

 mously developed in the Echidna) by observing, that the eye of the 

 aquatic Ornithorhynchus has a third valvular eyelid, the lens also being 

 more convex than in the Echidna. There is a lachrymal apparatus and 

 duct in the usual place. The nerves generally are upon the normal plan, 

 but the fifth nerve in the duck-billed creature is enormous, to supply 

 its curious mandible, which must possess extraordinary- sensibility, 

 though of a subdued kind, from its leathery covering, similar to that 

 of a delicate hand in a close-fitting glove. The large nasal branch of 

 the first division runs in a peculiar canal, but generally the distribu- 

 tion is as in other mammals, and it is sufficient if we say that six 

 nervous fasciae, generally very large, are distributed to the upper, and 

 four to the lower mandible, on each side. I am not sure that mention 

 is made in authors of two little organs or sacs, situate in the mouth, 

 upon the palate, and answering to the situation of the nostrils without ; 

 they must be palatal nares. Four rudimentary anterior teeth exist, in 

 addition to those commonly described, and are figured by Home. The 

 origin of this great fifth nerve, approaching in width to the nerve sup- 

 plying the lower extremity in man, is from the medulla oblongata, evi- 

 dently below the pons. The external auditory canal is long and wind- 

 ing in both animals, with a very wide opening in the Echidna. The 

 drum of the ear looks downwards in this animal, a little forwards and 

 outwards in the Ornithorhynchus, in which, too, it is smaller and longer ; 

 and it is stretched in both on a separate rim of bone like a tambourine. 

 Home and Blainville give only two bones to the internal ear, but we 

 also find the incus, which is four-sided, attached to the malleus, and 

 supporting the stapes, which is like a straight trumpet or neat nail in 

 form. The malleus is connected with the frame of the tympanum, and 

 also with its membrane. In the Echidna the Eustachian tube, also 



