4S2 ANATOMY OF VERTEBRATES. 



peculiarly so in the Manatees : the first in the Dugong has a long 

 oblique process from the under part of the neck, and a shorter 

 process, terminated by a rough surface, from the inner border, 

 two inches from the loAver end of the rib. The first three or four 

 pairs of ribs join, by cartilaginous ha3mapophyses, the sternum, 

 which consists of two bones and a xiphoid cartilage: the two 

 sternebers coalesce into a single bone, of the borders of which the 

 costal articulation occupy the middle third. From the third to 

 the sixteenth dorsal, the ribs are of nearly equal length. Many 

 of the succeeding ribs have a process from the posterior margin, 

 simulating the costal appendages in Birds. The metapophyses are 

 not developed so as to supersede the prozygapophyses, as in 

 Cetacea. The neural spines are of equal length and similar in- 

 clination slightly backward. In the Dugong, fig. 292 C, the atlas 

 has short par- and di- apophyses, and the neural arch is per- 

 forated on each side near its anterior border : in the axis the 

 transverse processes are chiefly by diapophyses. In the fourth 

 cervical, the right process was pierced by the vertebral artery ; in 

 the seventh, the left process : the other diapophyses were notched. 

 The centrum of the seventh cervical has a facet on each side 

 for the first pair of dorsal ribs. The side of the centrum 

 of the first dorsal vertebra bears two articular facets; one 

 of which is smaller than the other, looks forward, and receives 

 a part of the head of that rib which articulates with the pre- 

 ceding vertebra ; the other looks backward, and receives a large 

 share of the head of the second dorsal rib. The transverse 

 processes are long and strong, and present on their extremity 

 an articular facet which receives the tubercle of the first free or 

 dorsal rib. 



In Manatus Americanus the cervicals are also very short, but 

 only four of these compressed vertebrce intervene between the 

 axis and the first dorsal : seventeen vertebrae support the move- 

 able ribs, and are followed by about twenty-two lumbo-caudal 

 vertebrae : the hasmal arch commencing at the lower interspace 

 between the fourth and fifth of their series. The pelvic bones 

 are reduced, as in most Cetacea, to an ischium giving origin to an 

 * ischio-cavernosal,' and insertion to an ^ ischio-coccygeal ' muscle. 

 In a half-groA\Ti Manatee I have seen the neurapophyses of the 

 first twenty-nine vertebras still suturally joined to their centrum. 

 But two pairs of ribs join the sternum, which soon becomes a 

 single bone, with a costal process on each side of the middle 

 part. 



The vertebral characters o^ Rhytina agree in the main with those 



