550 Bulletin American Museum of Natural History [Vol. LXVI 



process, in this respect resembling inunguis, although the ventral malar 

 process in inunguis is always narrow and sharply tipped. 



The supraoccipital bones of senegalensis and manatus are of a com- 

 mon pattern but differ very strongly from the same bones in the Ama- 

 zonian manatee. The former present a very nearly flat, transverse plane, 

 whereas the latter are rugose and so mounded that the lambdoidal ridge 

 as seen from above is yoke-shaped. 



The outer borders of the exoccipitals of senegalensis are knotty, 

 pitted, and rough. In this respect the African species is similar to most 

 inunguis and unlike manatus. 



The foramen magnum of senegalensis and inunguis is roundish, 

 while in manatus it is oval. This shape is modified in part by differences 

 in the dorsal rim and also, as seen from the ventral surface, by the notch- 

 ing of the lower border. In manatees from the Congo, the Amazon, 

 Puerto Rico, and Guatemala, that is to say, in representatives of all 

 species, the dorsal border is strongly curved. Florida and Texas speci- 

 mens of manatus, however, have flat dorsal rims, perhaps one of the most 

 constant features on which the nominal race latirostris may lay claim to 

 subspecific individuality from manatus. The notching of the lower 

 border (the basioccipital) is also a fairly constant and peculiar mark of 

 the northern representatives of the species manatus. These differences in 

 the limits of the foramen may be partially synthesized by taking the 

 ratio of greatest vertical diameter to greatest horizontal diameter. Ex- 

 tremes are found in a specimen from the Amazon in which the vertical 

 diameter is 75 per cent of the horizontal, and in a Florida specimen in 

 which the corresponding percentage is but 54 per cent. There are no 

 specific limits to these ratios, however, for the Congo skull has a f oram- 

 inate index of 0.66; skulls of inunguis vary from about 0.65 to 0.75; and 

 specimens of manatus from 0.54 to 0.71. Within the species, however, 

 there seem to be geographic trends, for Florida specimens have indices 

 of from 0.54 to 0.61; a Texas specimen an index of 0.60; while four 

 Guatemalan manatees have corresponding indices ranging from 0.66 

 to 0.71 ; and a Puerto Rican manatee has an index of something over 0.70. 



Basally, skulls of the species of manatee may be recognized by a 

 number of characters. Over the occipitosphenoidal suture is an 

 eminence which in inunguis is median and simple, while in the other 

 species it is lateral and double, and anterior to this the shape of the 

 posterior nares is distinctive, though variable with age. In the Amazoni- 

 an manatees the opening is sagittate in young individuals, while in old 

 manatees it broadens out ventrally so that it is bicordate or presents the 



