552 Bulletin American Museum of Natural History [Vol. LXVI 



latter are of a smaller diameter and are strongly furrowed. In all 

 of the species there are anterior and posterior cingula, but though these are 

 smooth in the unerupted teeth of manatus and senegalensis, they are 

 deeply furrowed in inunguis. In the first two species each of the two 

 great transverse ridges is divided into three cones or sectors, but in the 

 Amazonian manatee these primary cones are more or less broken up 

 into a series of other smaller cones. 



The Mandible 



Hartlaub 's analysis of the characters of the mandible of senegalensis 

 is good, except that some of the features he assigned to the mandible 

 of inunguis and manatus do not well apply to the series of these species 

 that I have examined, and a few minor differences appear to have 

 escaped his notice. 



The interramal interval is broad in senegalensis as it is in manatus, 

 a feature by which these species are again easily distinguished from in- 

 unguis, and the rami of the latter species lie more nearly in parallel planes 

 than is the case in the other two species. The African species, however, 

 differs from manatus in that there is less of a constriction in the diameter 

 of the ventral border between the body and the angular process. In 

 senegalensis this process is wider and more in line with the ramus than 

 in the other species. 



The symphysial suture closes early in the African manatee, as 

 Hartlaub pointed out, and in this species there is no deep furrow along 

 the anterior margin. In this respect it differs from American manatees. 

 This furrow, which is most conspicuous in manatus, is a character best 

 developed at maturity and is not well marked in the newborn or very 

 young. 



The interior mental fossa is always deeper in young manatees than 

 in old, but comparing equal-aged material the fossae of African and 

 Amazonian manatees are, in most instances, deeper than those of Carib- 

 bean specimens. The character is not constant, however, and, contrary 

 to the conclusion of Hartlaub, is of little use to the taxonomist. 



The anterior end of the mandible of all species is essentially similar 

 in respect to form, as seen in norma dorsalis. Hartlaub claims that this 

 region is truncated in senegalensis and inunguis, whereas in the Surinam 

 manatee {manatus) it is tapered or tipped ; but my good series of manatus 

 shows no difference in this character from the other species, and I would 

 describe the anterior end of the mandible of manatus as truncated. 

 However, there is very often a sharp median cone of compact bone ex- 



