418 Dr. S. Woodward — On the Fossil Sirenia. 



It is interesting to observe, in confirmation of Dr. Murie's observa- 

 tion, that the skulls of Rhytina in the British Museum demonstrate 

 the former presence of small rudimentary incisor teeth in the 

 premaxillaries, two small alveoli being clearly shown ; and the 

 sides of these bones are swollen slightly, just where the pulp-cavities 

 of these small incisors would have been situated. 



As compensation for the absence of teeth in Rhytina, the palate 

 and sides of the gums of both the upper and the lower jaM' were 

 covered by tough corrugated horny plates, of peculiar structure, 

 which assisted in the process of mastication. 



With regard to the structure of the palatal and mandibular 

 laminae, although their function was undoubtedly that of the tritu- 

 ration of food, Prof. Brandt has shown ^ that they are destitute of 

 true bony or dental substance, and that they are in fact indurated 

 epithelium. 



Dr. Murie has also expressed his conviction that the strongly 

 ridged palatal plate in Rhytina is homologous with that found in 

 Manatiis and Ealicore. " It certainly," he adds, " does not appear to 

 me to be the representative of teeth, nor of the baleen plates met 

 with in the true Cetacea." " The maxillary alveolar ridges are 

 narrow and quite behind the bruising-plate, the latter occupying the 

 intermaxillary and not the maxillary bones " (Murie, Trans. Zool. 

 Soc. vol. viii. 1872, p. 167). 



Advantage was taken of the loose and readily separable state of 

 the sutures of the skull, to make a careful gelatine mould of the 

 brain-cavity. The result is shown in the cast exhibited in the Col- 

 lection, which differs somewhat from the figure (similarly obtained) 

 of the brain-cavity of Rhytina taken by Brandt. 



A comparison of the cast of the brain with that of other Sirenia 

 tends to the conclusion that the brain of Rhytina was intermediate 

 between that of Halicore and Manatus. It is interesting to observe 

 that the brain of Rhytina, considering the huge size of the body of 

 the animal, seems to be only one-sixth of the size of that of the 

 living Manatus or Halicore. This agrees with Prof. Marsh's obser- 

 vations on the smallness of the brain in Tertiary mammals,^ and is 

 strongly favourable to the very high antiquity of Rhytina. The plaster 

 cast of the brain of Rhytina exhibits traces of the small optic nerves, 

 and of a very large fifth hypophysis showing a rounded prominence.^ 



The bones of the ear of Rhytina Stelleri, namely, the os petrosum 

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

 sides of the skull. On removing the peat from the cavity of the 

 mid-ear, Mr. C. Barlow (the sihle formatore of the Geological Depart- 

 ment) discovered the three small auditory ossicles, namely, the Stapes, 

 Incus and Malleus, still within the cavity. They agree very closely 



1 Brandt, Mem. Imp. Acad. Sci. St. Petersbourg, 1846, vi. s^rie, pt. ii. Sci. 

 Nat. vol. V. livr. iv. pp. ]-160, tab. i.-v. 



2 See Marsh in Silliman's Journal, "On the small size of Brain in the Tertiary 

 Mammalia," 3rd series, vol. viii. 1874, p. 76, ibid. op. cit. vol. xii. 1876, p. 61 and 

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



3 Brandt, " Symbolse Sirenologicse," fasc. iii. 1878, p. 256. Tab. ix. Mem. de 

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



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