PERFECT SKELETO^• OF RHYTIXA GIG AS. 461 



separated from the cerebellum by a bon}- septum : he was unable to 

 find any other peculiarity. He proceeds : — " As no brain was pre- 

 served, the only means of knowing its form was by taking casts of 

 the cranial cavity with plaster of Paris. In general character the 

 brain of Rliytina is intermediate in form between those of Halicore 

 and Manatus. In some points perhaps it is nearer to the former, 

 but in others it approaches Manatus. The cerebral hemispheres 

 higher and more elongated than in the Manatees ; the posterior 

 lobes of the hemispheres longer and higher than the anterior ones, 

 convex above and impressed laterally, and the medulla oblongata 

 projecting above, in the form of a median longitudinal crest ; all 

 show a resemblance to the brain of Halicore. The hemispheres of 

 Rliytina are, on the contrary, shorter than in Halicore ; the anterior 

 lobes of the hemispheres of Rliytina are impressed on their anterior 

 faces and not convex as in Halicore ; again the medulla oblongata of 

 Rliytina is very broad, and the corpora clavata are more depressed 

 from the sides than in Halicore^ and remind one rather of Manatus. 

 But besides the above-mentioned affinities to Halicore and Manatus^ 

 Rliytina also shows peculiar characters of its own. 



"Thus the anterior lobes of the hemispheres of Halicore are 

 exceedingly convex ; in Rhytina they are even more so, and even 

 higher than in the Manatee ; they are depressed from the same 

 cause, on the anterior surface, especially on the sides, but less than 

 in Halicore and Manatus, but they overhang in the middle to a 

 greater degree. The posterior lobes of the hemisphere of Rliytina, 

 on their posterior border with the cerebellum, are broader than 

 in the other genera of Sirenians before mentioned (and in this 

 respect they are not unlike the same lobes in the elephant) ; they 

 also show on the upper surface an oval convexity or furrow ; these 

 then constitute peculiarities. The hemispheres of the brain of 

 Rhytina further differ in this, that they appear to approximate very 

 closely to one another. The cerebellum in Rhytina is depressed 

 to a greater extent, and is broader and stronger than in the other 

 genera. The brain in Rhytina affords therefore a form intermediate 

 between Halicore and Maiwtus. Otherwise the brain of the Rliytina, 

 considering the huge size of the body of the animal, seems to be six 

 times smaller * in proportion than that of Manatus or Halicore. 

 The plaster-cast of the brain of Rhytina shows traces of the small 

 optic nerves, and of a very large fifth hypophysis showing a rounded 

 prominence "t. 



Bones of the Ear o/Ehytina ^tQ)leT\ preserved. — The os petrosum 

 of the periotic, with the tympanic annulus, is preserved on both the 

 right and left sides of the skull. 



On removing the peat from the cavity of the mid-ear Mr. C. 



* This agrees with Prof. Marsh's obsei'vations on the smalhiess of the brain 

 in Tertiary mammals, and is in favour of the very high antiquity of Bhytina. 

 See Marsh in Silhman's Journ., "On the small size of Brain in the Tertiary 

 Mammaha," orcl series, vol. viii. 1874, p. 66, ibid. op. cit. vol. xii. 1876, p. 61 

 and vol. xxix. 1885, pp. 190-193. 



t Brandt, " Symbolas Sirenologicae," fasc. iii. 1878, p. 256. Tab. ix. Mem. de 

 I'Acad. Imp. d. Sc. St. Petersbourg, ser. vii. tom. xii. 



