460 DE. H. -WOODWAED ON AN ALMOST 



broadest ; the supraoccipital portion is very rugose, the condyles are 

 semicircular and prominent, and the foramen magnum is very wide. 



The Lower Jaw. — The lower jaw is deep in proportion to its 

 length. The coronoid process rises very little above the condyle 

 itself. The symphysis of the mandible extends for about one third 

 (or rather more) of its length, having a convex contour on its 

 upper surface to correspond with the concave contour of the pre- 

 maxilla. The symphysial surface is very rugose. 



T^eeth absent or only Rudimentary Incisors. — Two kinds of teeth 

 (molars and incisors) are usually present in most of the Sirenia *. 

 Dr. James Murie, in his elaborate and exhaustive memoir on the 

 Manatee (Trans. Zool. Soc. vol. viii. 1872, pp. 127-202, pi. xvii.- 

 xxvi.), observes: —"Although ETiytina was edentulous in the adult 

 condition, I strongly suspect that, like other Sirenian genera, rudi- 

 mentary teeth may have existed in its earlier stages of growth. Xord- 

 mann seems also favourably inclined to this opinion." 



Trace of Rudimentary Incisors. — It is interesting to observe, in 

 confirmation of Dr. Murie's observation, 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 smaU incisors would 

 have been present. 



Horny Palate. — As compensation for the absence of teeth in 

 Rhytina., the palate and sides of the gums of both the upper and 

 the lower jaw 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 t 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 

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

 me to be the representative of teeth, nor of the haleen plates met with 

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

 and quite behind the Iruising -plate, the latter occupying the 

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

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



Cast of Brain-cavity secured. — 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, which differs somewhat from the figure 

 (similarly obtained) of the brain-cavity of Rhytina taken by Brandt. 



Brandt cited on Brain of Ehytina. — That author observes that, 

 according to Steller, Rhytina has a small cerebrum, which is not 



* In Frorastomus canines are also developed ; but Ehytina possessed neither. 

 t Brandt, Mem. Imp. Acad. Sci. St. Petersbourg, 1846, vi. serie, pt. ii. Sci. 

 Nat. vol. V. livr. iv. pp. 1-160, tab. i.-v. 



