EOSIREX LIBYCA. 
203 
be expected in a land-animal, the olfactory lobes are more developed than in 
Kosircii, bnt this does not affect the general similarity, wliich is the more striking wlien 
taken in conjunction with the many other points of likeness existing between the Sirenia 
and the Proboscidea. It is interesting to note further that in the Middle Eocene forms 
of botli those orders the brain is relatively largo for mammals of that early date. In the 
terrestrial Proboscidea living exposed to many enemies, and in a complex and changing 
environment, this large development of the brain persists even in the modern types, 
and, indeed, their survival may be due to this very character. On the other hand, in 
the Sirenia, which early adopted a sheltered and sluggish mode of life, the conditions 
of which must have remained nearly constant, although there is some tendency towards 
increase in the size of the brain, there is no need for increased complexity, the result 
being the peculiar type of structure which Dr. Elliot Smith states can only be 
parallelled by the structure “ occasionally ])resented by the brains of idiots in which 
the process of elaboration has ceased in the earlier mouths of intra-uterine life, and 
the organ has simply grown in size without becoming perfected in structure.” 
Upper Dentition . — The dental formula of the upper jaw appears to be i. 2 or 3, 
c. 1, m.-\-pm. 8. At the anterior end of the downwardly turned rostrum there are 
the alveoli for a pair of enlarged incisors (^. 1), which were oval in section and directed 
downwards and foinvards ; the sockets are comparatively shallow, these teeth being 
apparently rooted. 
The only trace of the second incisors [i. 2) seems to be a small alveolar pit situated a 
little below and just in front of the third incisors {i. 3), which also are represented in this 
specimen by their alveoli only, these being placed immediately in front of the maxillo- 
premaxillary suture and on the side of the rostrum about 5 mm. above the sharp 
edge of the premaxillae. Both the second and third incisors are clearly on the way 
to disappearance, and seem to have been thrust outwards to the side of the jaw 
by the development of a horny plate covering the palatal and sharp lateral edges 
of the premaxillary region and replacing the teeth functionally. The canine 
alveolus (c.) is also small and on the side of the rostrum ; it is placed immediately 
behind the maxillo-premaxillary suture, and is abnost in contact with the alveolus 
of i. 3. 
Behind the canine, and separated both from it and from one another by nearly 
equal intervals (about 1‘3 cm.), are the sockets for four single-rooted teeth. In the 
type specimen only the last of these is in situ-, it consists of a comparatively small 
single cone with a very strongly developed internal shelf or cingulum. Immediately 
behind this come the relatively small quadrate two-rooted molars. There are four of 
these teeth, of approximately equal size. Each molar crown consists of two 
transverse ridges, each composed of two pointed cusps without any secondary 
tubercles ; there is also a small anterior ridge or cingulum with a median cusp, which 
in wear becomes connected with the antero-internal main cusp. All the specimens 
2 D 2 
