NO. 2113. EXTINCT SIRENIAN DESM0STYLU8 HESPERUS— HAY. 397 



DESMOSTYLUS Marsh. 



A Miocene genus of the Sirenia, exhibiting in its skull many primi- 

 tive characters. Snout little deflected, nasal opening small and far 

 in advance of the orbits. Nasal bone long and narrow. Brain case 

 broad and rounded above; the temporal ridges feebly developed. 

 Upper jaw with a pah- of tusklike teeth, probably canmes. Lower 

 jaw with two pairs of tusks. Molars and some of the premolars 

 composed each of high, closely appressed columns, varying in number. 



Species of Desmostylus. 



1. A species of moderate size, apparently with somewhat broader snout, higher skull, 



and narrower palate posteriorly. First upper molar apparently with five col- 

 umns. American., hesperus. 



2. A large species, with a snout apparently narrower, a frontal region lower, palate 



posteriorly wider, and the first upper molar with eight columns. Japanese. 



w(x tds&i 

 EXPLANATION OF PLATES. 



Explanation of numerals: 1, supraoccipital; 2, parietal; 3, frontal; 4, nasal; 5, 

 premaxillary; 6, maxillary; 7, lachrymal; 8, squamosal; 9, zygoma; 10, jugal; 'll' 

 capsule for second molar; 12, nasal opening; 13, exoccipital condyle; 14, basioccip'ital; 

 15, basisphenoid; 16, adventitious bone; 17, mastoid; 18, palatine; 19, tympanic 

 bulla; 20, supposed third premolar; 21, fourth premolar; 22, first molar; 23, second 

 molar; 24, foramen lacerum posterius; 25, condylar foramen; 26, stylomastoid for- 

 amen; 27, tympanic ca\dty; 28, infraorbital foramen; 29, exoccipital; 30, paroccipital 

 process; 



Plate 56. 



Desmostylus hesperus Marsh. Skull seen from above. X f . 



Plate 57. 



Desmostylus hesperus Marsh. Skiill seen from below. X f . 



Plate 58. 

 Desmostylus hesperus Marsh. 

 Figs. 1, 2. Skull. X |. 



1. Seen from the left side. 



2. Seen from behind. 



Figs. 3, 4. Supposed upper right second molar. X 1. 



3. Showing grinding surfaces. Anterior end directed upward. 



4. Seen from the left side. Anterior end directed to the left. 

 Figs. 5, 6. Supposed upper fourth premolar. X 1. 



5. View of grinding surface. 



6. View of tooth from the rear. 



