THE PECTORAL LIMB 



marked extent, being thrust to the side for making sharp turns or 

 waved about as assistants to the accomplishment of a variety of rather 

 languid evolutions. In addition they are said to be habitually employed 

 for bringing herbage toward the mouth, and it is frequently stated that 

 the females use the flippers for clasping the young. 



Externally the manus of the Sirenia is paddle-like though of a rather 

 irregular and blunl form, rather than gracefully shaped like that of the 

 sea-lion and most whales. I would not regard any of the known stimuli 

 as particularly strong for the assumption of a fore limb of this char- 

 acter'and it seems likely that an immense space of time has been neces- 

 sary for its attainment. Small blunt nails are present apparently in all 

 forms of the manati except the African Trichechus inunguis, and it 

 is interesting to note that these are situated upon the flipper border where 

 they would be most available for such acts as scratching, although they 

 hardly project sufficiently to be very useful in this respect. Nails are 

 absent in Halicore as was probably also the case in Hydrodamalis. 



The synovial character of the wrist joint seems to be completely re- 

 tained in the Sirenia. As these mammals are entirely helpless out of 

 water there is not the need for abduction to 90 degrees of the wrist 

 that the sea-lion has, but yet this joint is said to be definitely and quite 

 surprisingly mobile, probably because of feeding needs. The pisiform 

 bone is absent in this order. In the dugong the carpal elements are 

 astonishingly reduced to three, one subtending the radius, a second the 

 ulna, and a third elongated bone distad with which the first four meta- 

 carpals articulate. In the manati there are six carpal elements (Flower 

 said seven) arranged in two transverse rows of three each. The sig- 

 nificance of these carpal details is unknown. 



The sirenian metacarpal and digital bones are somewhat flattened, 

 especially in the manati, as is usual in aquatic mammals, the ungual 

 phalanges are of irregular shape and particularly flat as in the sea-lion, 

 and the pollex is definitely reduced. The fourth digit is the longest, 

 this representing the tip of the flipper. Beddard (1900) has mentioned 

 that in this order "hyperphalangy is also met with but to a very small 

 extent." In most skeletons any predigital cartilaginous elements which 

 might be present would be lost during cleaning, but in the mounted 

 dugong in the National Museum the left manus (fig. 35) shows a small 

 cartilaginous nodule, now shrunken, upon the tip of the third digit, this 

 being the fourth phalangeal element. Furthermore this is of the exact 

 character as the single premetatarsal element of the first, and the third 

 of both the second and fifth digits, which is of significance in show- 



[247] 



