XII.] SIRENIA. 20I 



vacuity, between the exoccipital and alisphenoid, common 

 to all skulls, is of immense extent. 



The frontals (.^r) are narrow, and run backwards between 

 the parietals to the upper part of the cerebral fossa of the 

 brain cavity, and forwards a short distance over the nasal 

 cavities ; each is produced anteriorly into a long narrow 

 process, inclining outwards and downwards, between the 

 temporal fossa behind, and the great anterior narial openings 

 in front, forming the roof of the orbit. This cavity has a 

 very prominent margin, especially below and in front, where 

 it is formed by the very largely developed malar. This bone 

 sends upwards a conspicuous postorbital process, which 

 nearly (in some cases completely) meets that of the frontal, 

 and then is continued below the zygomatic process of the 

 squamosal as far as the wide shallow glenoid fossa, and 

 sends down from its middle a broad flattened process with a 

 thickened and rough inferior border. There is a very small 

 scalelike and imperforate lachrymal in the usual situation at 

 the anterior and inner angle of the orbit. The antorbital 

 foramen of the maxilla is very large. 



One of the most peculiar features cf the upper surface of 

 the face is derived from the position of the anterior nares, 

 which is a further modification of that met with in the Tapirs 

 among the Ungulata, and presents some approach to that 

 so characteristic of the Cetacea. Taken together they form 

 a large lozenge-shaped aperture, which extends backwards 

 considerably behind the orbits. Their sides are formed by 

 the ascending processes of the premaxillae below, and by 

 the supraorbital processes of the frontals above, no trace of 

 nasals being found in most skulls, though these bones are 

 occasionally present in a most rudimentary condition attached 

 to the edge of the frontals, far away from the middle line, 

 a condition quite unique among the Mammalia, or only 



