228 THE SKULL. [CHAP. xir. 



Cetacea, and is formed of the same dense heavy bone as the 

 rest of the skull. The rami are firmly united by a symphysis 

 (s) of moderate extent in front, and diverge widely behind. 

 The posterior border is of considerable vertical depth, the 

 condyle (cd\ with its obliquely placed, oval, convex arti- 

 cular surface, being raised high above the horizontal alveolar 

 border. The coronoid process (cp) is large and directed for- 

 wards. The angle (a) is well marked, thickened, and some- 

 what inflexed, but does not form a distinct process. The 

 lower border of the ramus is very thick, rounded from 

 side to side, and concave from before backwards. The 

 symphysial portion is compressed laterally, but its upper 

 surface forms a somewhat expanded, rugose surface, concave 

 in the middle line, to which a horny plate is attached in the 

 living animal 



The anterior arch of the hyoid has a single, slender, 

 slightly-curved bone (stylohyal), three inches long, cylindrical 

 at its upper end, and laterally compressed below, attached 

 above by a broad, short ligament, chiefly to the exoccipital, 

 but also to the squamosal and tympanic. The basihyal 

 is a broad, flat, reniform plate, and the thyrohyals are not 

 ossified. 



In the other existing Sirenian, the Dugong (Halicore), from 

 the Indian Seas, the skull resembles that of the Manatee in 

 its essential characters, especially the form of the brain-case, 

 the condition of the tympano-periotic bones, and the form 

 and situation of the anterior nares ; but it differs mainly in 

 the great development of the premaxillary bones, which curve 

 downwards in front, and lodge large descending tusks The 

 deep, compressed symphysial portion of the mandible is 

 bent down in a corresponding manner. The zygoma is less 

 massive, the orbit is not closed behind, and the lacrymal 

 bone is more developed. The nasals are absent or quite 

 rudimentary. 



