352 THE VERTEBRATE SKELETON. 



which includes the living sloths, anteaters, armadillos, pangolins 

 and aard varks, together with various extinct forms, chiefly 

 found in beds of late tertiary age in both North and South 

 America, the best known being the Megatheridae and Glypto- 

 donts. 



Order 2. SiRENiA 1 . 



The skeleton of these animals has a general fish-like form, 

 in correlation with their purely aquatic habits. The fore limbs 

 have the form of paddles, but the number of phalanges is not 

 increased beyond the normal. There are no external traces of 

 hind limbs. 



The whole skeleton and especially the skull and ribs is 

 remarkably massive and heavy. The dentition varies ; in the 

 two living genera Manatus and Halicore, incisor and molar 

 teeth are present, in one extinct genus, Rhytina, teeth are 

 entirely absent, while in another, Halitherium^ the dentition 

 is more decidedly heterodont than in living forms. In the 

 two living genera the dentition is monophyodont, but in 

 Halitherium the anterior grinding teeth are preceded by 

 milk teeth. The tongue and anterior part of the palate and 

 lower jaw are covered with roughened horny plates. The 

 skull is noticeable for the size and backward position of the 

 anterior nares, also for the absence or small size of the nasal 

 bones. There is no union of certain of the vertebrae to form 

 a sacrum, and in living forms the centra are not terminated by 

 well-formed epiphyses 2 . 



The cervical vertebrae are much compressed, but they are 

 never ankylosed together. In Manatus there are only six 

 cervical vertebrae. The caudal vertebrae have well-developed 

 chevron bones. The humerus is distinctly articulated to the 



1 See J. F. Brandt, Symbolae Sirenologicae, St Petersburg, 1846, 1861, 

 1868. 



2 Epiphyses are fully developed in Halitherium, and traces occur in 

 Manatus. 



