ORDER SIRENIA. 



1309 



(fig. 1 180); the eyes minute; and the small ear has no external 

 pinna. The bones of the skeleton are extremely dense and solid ; 

 the cervical vertebrae may be reduced to six, but are never anchy- 

 losed together ; and the axis has a well-developed odontoid process. 

 The vertebrae have no epiphyses ; and none of them unite to form 

 a sacrum. Large chevron-bones are present. Clavicles are want- 

 ing ; and no recent forms show any trace of the femur, although a 

 styliform rudiment of this bone is found in the extinct Halitherium. 

 The two bones of the forearm in the existing genera (fig. 1 1 8 1 ) are 

 subequal in size, and frequently anchylosed together. In recent 

 forms the skull (fig. 1182) is remarkable for the upward direction 



Fig. 1 182. — Right lateral view of the skulls of Halicore (a) and Manatus (b). Much reduced. 



of the narial aperture, and for the absence or rudimental condition 

 of the nasals. The teeth can only be differentiated in the living 

 genera into incisors and cheek-teeth, which are separated from one 

 another by a long interval ; but in one fossil genus there is a more 

 complete dental series • while in Rhytina these organs were totally 

 wanting. In the existing genera the dentition is monophyodont, 

 but milk-molars occur in Halitherium. This order is known from 

 the Eocene upwards, but appears to have been steadily dying out 

 to the present period. The fossil forms, although decidedly more 

 generalised than their living representatives, do not at present afford 

 any clear indication as to the origin of the order. 



Family Halicorid^e. — In this family the one existing genus 

 Halicore (Dugongs) has the deflected premaxillae (fig. 1182, a) fur- 

 nished with a pair of large tusk-like incisors ; and there are also 

 four pairs of functionless germs of cutting-teeth in the similarly de- 

 flected mandibular symphysis. Five or six cheek-teeth are devel- 

 oped in each jaw, which are usually cylindrical, and all grow from 

 persistent pulps, and are not coated with enamel. The Dugongs 

 inhabit the coasts of the Red Sea and the Indian Ocean. Prohali- 

 core, from the Pliocene of France, is regarded as nearly allied to the 

 existing genus. The only other form which has been referred to 

 this family is known by a molar from the Tertiary of California, 

 which has been described by Professor Marsh under the name of 



