PEEFECT SKELETON OF EHYTINA GIGAS. 463 



animals in keeping their large bodies sunk beneath the surface of 

 the shallow waters in which they dwelt whilst feeding upon the 

 marine vegetation upon which they wholly subsisted. 



Variations to the rule of seven Cervical Vertehrce usually obtaining in 

 the Mammalia.- — Although the normal number of cerYical vertebras 

 maintained in the Mammalia is usually seven, yet some of the 

 Sirenia (such as the "American Manatee") have only six. Con- 

 versely in Bradypus the number of the cervical vertebrae is increased 

 to eight or nine. This is explained by the fact that the thoracic 

 vertebrae in Brady pus pass into the cervical region, while the 

 diminution to six in Choloepus and in the American Manatee is 

 similarly explained by the complete development of the rib of the 

 seventh cervical vertebra. 



Bhytina has been described by Steller as only possessiug six 

 cervical vertebrae, like the Manatee ; but Brandt correctly gives the 

 number as seven, and the specimen now in the Museum confirms 

 this determination. 



The atlas- and axis-vertebrae in Bhytina are fairly robust, 

 and the atlas is as broad as the second dorsal vertebra ; but the 

 five remaining cervical vertebrae, although quite free, are thin and 

 plate-like, as in the Cetacea proper. But the Sirenia are distinguished 

 from the true Whales by their capability of moving the head from 

 side to side, and up and down, by means of the " odontoid " process 

 of the axis vertebra on which the head rotates. In the Cetacea, in 

 which the cer\ical vertebrae are anchylosed together to a greater or 

 less, extent, and the neck is consequently immovable, the odontoid 

 process is also wanting. 



As the Sirenia spend their whole lives browsing upon the Lami- 

 narice and other AJgae and aquatic plants, this power to move the 

 short neck pretty freely must be essential to them both in feeding 

 and also in putting up their heads to breathe. 



The number of vertebrae attributed to the Sirenia, both of living 

 and extinct genera, is very variable according to difi'erent authors. 

 Prof. Brandt attributed to Bhytina 7 cervicals, 19 thoracic or dorsal 

 vertebrae, and from 34 to 37 lumbar, sacral, and caudal. The 

 cervicals and dorsals are readily determined ; but, as none of the 

 vertebrae are anchylosed together to form a sacrum, it is a matter of 

 some difficulty to decide which are lumbar and which are sacral 

 vertebrae. 



Not only does anchylosis never occur in the vertebrae of the Sirenia, 

 but the fiat ends of the centra of the vertebrae do not ossify separately 

 so as to form disk-like epiphyses in the young state, as is commonly 

 the case in all the other Mammalia. 



Brandt indicates the 7th vertebra beyond the last of the dorsal 

 or thoracic series as bearing the rudimentary pelvis ; but as the ver- 

 tebrae are never anchylosed to form a sacrum, we can only conjecture 

 (by noticing a slight prominence upon the posterior border of the 

 extremely wide transverse processes) which of these lumbar-sacral 

 vertebrae seem marked as sacral, probably about the 5th, 6th, and 

 7th. The 13 vertebrae next behind the dorsal series may, from their 



