204 THE SKULL. [chap. xii. 



cular surface, being raised high above the horizontal alveolar 

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

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

 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 some-] 

 what expanded, rugose surface, concave in the middle line, 

 to which a horny plate is attached in the living animal. 



In a perfectly adult West Indian Manati (Af. australis), 

 in the Leyden Museum, the anterior arch of the hyoid has 

 a single, slender, slightly curved bone (stylohyal), three 

 inches long, cylindrical at its upper end, and laterally com- 

 pressed 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 Manati 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 lachrymal 

 bone is more developed. The nasals are absent or quite 

 rudimentary. 



