PEOIT. OWEN ON PllOEASTOMUS SIKENOIDES. 565 



The longer and deeper prEemaxillaries accord with the indications 

 of their supporting incisor teeth in Prorastomus. The minor relative 

 depth and length of these hones in Manatus are associated with the 

 minute size of the single incisor. The mandible is longer in propor- 

 tion to its depth, and also in proportion to the rest of the skull, in 

 Prorastomus than in Manatus. 



Felsinotherium differs from Prorastomus in the greater extent of 

 the downbent fore part of the prsemaxillaries, which, as in Halicore 

 and in Bhytiodus, is in relation to the deep socket for the single large 

 deflected tusk representing the incisive part of the dental formula. 

 The praemaxillary and corresponding part of the mandible continue 

 in Prorastomus the horizontal line of the skull, in relation to the 

 three small subequal incisors which are carried on each side of both 

 upper and lower jaws. This is the most marked feature in which 

 Prorastomus adheres to the normal mammalian type, while showing 

 the essential characters of the marine herbivore. 



Whether the angle of the jaw is produced downward in the degree 

 in which it is in Manatus, and especially as in Felsinotherium, is 

 undeterminable, as that part is broken off in Prorastomus ; but this 

 very fracture makes it probable that the Sirenian mandibular cha- 

 racter was present in some degree. 



In the few comparable parts of the base of the skull the chief differ- 

 ence between Prorastomus and other Sirenia, fossil and recent, is in the 

 smaller relative size of the petrotympanics (PL XXVIII. fig. 3, 16) ; 

 they are, as usual, exposed at the large vacuities on each side the 

 basioccipito-sphenoid (ib. 1, 5) characteristic of the order. 



In Prorastomus the breadth of the skull outside the zygomatic 

 arches is half the length of the skull ; in Felsinotherium it is less 

 than one half that length. 



The upper tract of the cranium in Prorastomus contracts ante- 

 riorly to the temporal apertures ; in Felsinotherium it rather expands 

 as it approaches the postorbital prominences. 



The zygomatic arches, deep and massive in both Sirenians, are 

 more outwardly arched in Prorastomus j they run straight and pa- 

 rallel with the supero-lateral ridges of the cranium in Felsinotherium. 



The outer nostril in Felsinotherium extends back to the parallel of 

 the hind border of the orbit ; in Prorastomus it does not quite attain 

 the anterior border of the orbit. 



In Felsinotherium the continuation of the zygoma along the 

 lower border of the orbit is in a line with the rest of the arch, and 

 forms a right angle with the maxillary part where this passes 

 inward to join the body of that bone. In Prorastomus the maxillary 

 part of the orbital border passes more gradually inward and upward 

 to the body of the bone. 



The external nostril, though horizontal and longitudinally ellip- 

 tical in Prorastomus, is shorter in proportion to the skull and to its 

 breadth than in Felsinotherium and recent Sirenia. 



In all these differences Prorastomus, while retaining the essential 

 Sirenian characters, departs less from the normal forms of the mam- 

 malian skull than does Felsinotherium and other known extinct 

 Sirenians, and still less than do the existing species. 



