NO. 2113. EXTINCT 8IREN1AN DESM0STYLV8 HESPERUS— HAY. 385 



DESMOSTYLIDAE, new family. 



Diagnosis of family. — Snout and lower jaw elongate and only 

 slightly bent downward. Nasal opening small and far in advance of 

 the orbits. Brain case more inflated behind and not compressed in 

 front, and the temporal ridges feebly developed. Upper jaw with 

 one pair of possibly latent tusks; lower jaw with two pairs of hori- 

 zontal protruding tusks. Molars high crowned, composed of two 

 longitudinal rows of appressed columns, with sometimes one or more 

 intercalated columns. 



The only genus at present known to belong to the family is Des- 

 mostylus, the diagnosis of wiich must be the same as that of the 

 family, as above given. To this genus belongs the type Desmostylus 

 liesperus Marsh, here to be described, and the Japanese species to be 

 named below. 



As will be seen from the illustrations (pis. 56-58), the skull is elon- 

 gate, the snout rather narrow and prolonged, but truncated in front. 

 The undulating surface of the occiput makes slightly less than a right 

 angle with the axis of the skull. Along the midline, beginning behind, 

 the surface is convex over the brain case, concave behind the orbits, 

 then again convex to the descending snout. Viewed from above 

 there appears in front of the orbits no such sudden contraction in 

 width as is. seen in Trichechus and Halicore. On the left side the con- 

 traction seems to be more abrupt because of the lack of some bone. 

 According to Andrews' figures, the contraction in the width is abrupt 

 and considerable in Eosiren; while in Eotherium, although the reduc- 

 tion in width is less sudden, it continues nearly to the front of the 

 snout. Apparently the center of the orbit in Desmostylus is very 

 nearly halfway from the occipital crest to the front of the snout. 

 This was the position of the orbit in Prorastomus and apparently also 

 in Eotherium, both belonging to the Eocene. 



The supraoccipital (pi. 58, fig. 2) joins the exoccipitals below and 

 the parietal above. Its outer angles barely come into contact with 

 the squamosal. In the lower half of the midline of the supraoccipital 

 there is a shallow groove; in the upper half a slight ridge, which 

 upwardly expands into the occipital crest. This crest is formed prin- 

 cipally by the supraoccipital as far as the latter extends. On each 

 side of the ascending ridge the bone is slightly excavated. It does 

 not enter into the boundary of the foramen magnum. 



The exoccipitals and the basisphenoid form a single mass. The 

 suture in front of the latter is yet open. The exoccipitals meet above 

 the foramen magnum a distance of 12 mm. The occipital condyles 

 are sessile. The foramen is wider than high and is notched neither 

 above nor below. Laterally, the exoccipital comes into contact with 

 81022° — Proc.N.M.vol.41>— ] 5 25 



