98 Walter Stanley: 



" The Manatees (^eiius Manatus), of which there are two well-known 

 forms, one inhabitin<r the West Coast of Africa, and the other the 

 East Coast of Central and South America, never have more than six 

 vertebrae in the cervical region." 



'' In a specimen of the Manatus senewalensis, in the Museum of the 

 Collefie of Surjifeons, the second and third are ankylosed by their 

 bodies. In the skeleton of M. Americanus, in the Museum of Cam- 

 bridge, the sixth cervical vertebrae carries a distinct moveal)le rib." 



Flower does not explain why only six bones occur in the neck of 

 the manatee, but it is quite clear that the details he describes point 

 most strongly to the fact that the normal seventh cervical vertebrae 

 of mammalia has in the manatee developed a paii' of perfect cervical ribs. 



The evidence in support of this is open to no other interpretation, 

 especially in face of the fact that the specimen at Cambridge has 

 extended the rib-forming process as far as the sixth bone, and also in 

 face of the added fact that the specimen of the manatee in the Mel- 

 bourne Museum shows upon the body of the six vertebra demifacets 

 for the head of the rib that rests in an articulation formed by the 

 sixth and seventh bones. It also seems that as the miocene 

 Halitherium was true to uuimmalian type, the manatees have acquired 

 the change to the six neck bones. 



TTie Duciotui occupies a variable position in regard to cervical 

 ribs ; usually this animal is without cervical ribs ; the specimen in 

 the Melbourne Museum has a pair of short cervical ribs upon the 

 seventh cervical vertebra ; they are about two inche<s long, and they 

 form movable articulations. All the bones of the neck of the dugong 

 are flattened, and they are not fused by ankylosis. 



It is not as easy to ascertain the cause of the development of 

 cervical ril)s in manatees and dugongs as it is in man, for in these 

 animals it is necessary to consider two possil)le causes, and then to 

 determine which is the causative factor. It is therefore necessary 

 to discuss whether the manatee develops its cervical ribs to stiffen its 

 neck area, as fish structure suggests, or whether the cervical ribs of 

 the sirens develop for respiratory purposes. If material could be 

 readily procured for dissection, the matter could easily be cleared up, 

 but even at Port Darwin a specimen of a dugong for investigation is 

 most difficult to procure. If the manatee required a stift'er neck than 

 it has, impulses would probably set up an ankylosis such as that 

 which exists in the porpoise ; the ankylosis that is already 

 established between the second and third bones, places it beyond 

 doubt that the manatee can, under suital)le impulses, set up ankylosis 

 of its neck bones. 



It is unlikely that the manatee uses twn inoaiis to stitiVn its nwk, 

 ankylosis and cervical rib formation, when by aiikvli>sis alone the 



