70 THK VOYAGE OF H.M.S. CHALLENGER. 



Trichechus (Odobasnus), Linnaeus. 



Horizontal part of premaxilla very short and thick, and with a strong tubercle ; 

 ascending part articulating with nasal. Superior maxilla swollen in front, and with 

 large alveoli for lodgment of canine tusks ; these alveoli are in the transverse plane of 

 the front of the face. Anterior nares relatively small, vertical in direction, and with 

 scarcely any premaxilla in front of the opening. Occipital crests distinct and meeting 

 mesially. No sagittal crest. Crests on frontal and temporal hones marking limit of 

 temporal muscle. A slight ridge along line of squamous suture. Interorbital and 

 interzygomatic region of frontal not very constricted. Hard palate truncated imme- 

 diately in front of strongly projecting hamulars and in line with anterior border of 

 glenoid fossae ; transverse part of palato-maxillary suture well behind the posterior edge 

 of maxillary root of zygoma ; vomer entirely concealed by hard palate, its posterior 

 border articulates with vomerine crest of palate considerably in front of the truncated 

 border. Zygomatic arch not bulging, its greatest width is at the zygomatic process of the 

 temporal, and the interzygomatic j>art of the skull is much below the greatest width of the 

 cranium ; zygomatic process of temporal short and not nearly reaching the orbital process 

 of the malar. Orbits relatively small ; infraorbital foramen large and opening below the 

 floor of the orbit. Tympanic bulla not swollen, but uniformly roughened on inferior surface 

 and continuous with the short thick wall of the external auditory meatus, the mouth of 

 which looks directly outwards. Foramen lacerum posterius relatively small. Par- 

 occipitals not detached processes. Basi-occipital not perforated mesially. Occipital 

 condyles converging anteriorly and sometimes continuous. Condyloid foramen imme- 

 diately in front of condyle. 



Permanent dentition- 



3-3 

 3-3 



5 — 5 

 Milk dentition — in. - — , c. - — , p. c. - — = 34. 

 3 — 3 1-1 4-4 



The last and penultimate upper molars and the last lower molar often rudimentary or 



absent. Mandible with a massive, incurved, subcondyloid tubercle separated by a notch 



from the feeble angle ; condyle with scarcely any neck, coronoid short and broad, sigmoid 



notch very shallow, lower border of body everted and with a wide arch ; symphysis 



ankylosed. Pineal body remarkably developed. 



