384- I'JiOVlJEDINQS OF THE NATWXAL MUSEUM. vol.19. 



Width of skull across zygomatic arches, at front of the glenoid fossce 210 



Width of skull at front of zygomatic processes of the temporal bone 190 



From front of one orbit to front of the other 93 



Width of snout at the rear of the nasal opening 77 



Width of hinder nares 54 



Width of palate between second molars 65 



Width of palate between front premolars 68 



It seems best, first of all, to compare briefly Desmostyhis with other 

 sirenians. From all existing forms it differs in having the snout 

 little bent down on the axis of the cranium (pi. 085 fig. 1). Yoshi- 

 wara and Iwasaki state that in their specimen the jaws showed no 

 trace of a downward curvatm-e, but one has only to view their plate 2 

 to see that there is a downward flexure of the face about halfway 

 between the tip of the snout and the orbit. However, in Owen's 

 Prorastomus, from the Eocene of Jamaica, the jaws appear not to 

 have been bent at all downward. In Desmostylus the flexure so 

 commonly found in sirenians is just beginning to manifest itself. It 

 appears to be somewhat more strongly developed in D. liesperus than 

 in the Japanese form. Another feature of Desmostylus which dis- 

 tinguishes it from most of its kindred is the small size and the 

 anterior position of the external narial opening. This is about the 

 size of the orbit, and has its anterior border a short distance behind 

 the front of the jaw and far in advance of the orbits. In a specimen 

 of manatee from Demarara River it begins quite as far in front, but 

 it extends far behind the orbit and is equal to nearly one-half of the 

 length of the skull. In Prorastomus the nostril is about the size of 

 the orbit, but its rear comes nearly to the front of the latter. In the 

 Eocene sirenians of Egypt, Eotherium and Eosiren, as described and 

 figured by Andrews,^ the external nasal opening is rather large and 

 extends back to or beyond the front of the orbit. 



In Desmostylus the nasals are less reduced than in other known 

 sirenians (pi. 56) . The brain case is less compressed than in most 

 su-enians and there are very feebly developed temporal ridges. 

 Something like this condition seems to be found in Eotherium.^ The 

 teeth of Desmostylus differ greatly from those of other Sirenia. In 

 the lower jaw there are two pairs of tusks.^ The molars consist of a 

 varying number of closely appressed columns which spring from the 

 base of the crown. 



In all the ways mentioned in which this skull differs from that of 

 other described sirenians, except the teeth, it is more primitive than 

 the hitherto described forms, except probably Prorastomus. Evi- 

 dently the animal will enter none of the families hitherto proposed. 

 A separate family must be constituted for it, and naturally it must 

 take the name 



> Andrews, Cntalopue ol the Tertiary Vertebrata of the Faj-uin, Egypt, p. 20t".. 

 « Yoshiwara and I\va,saki, Journ. Colloge Scien., Imp. Univ. Tokyo, vol. 10, p. 5. 



