386 PROCEEDI^'Gii OF THE NATIONAL MUSEUM. vol. 49. 



the upper hinder angle of the squamosal for a distance of 28 mm.; 

 below this it joins the mastoid portion of the petrosal bone apparently 

 to the lower end of the paroccipital process. These processes prob- 

 ably extended down to the level of the lower surface of the occipital 

 condyles; but the latter are slightly eroded, the former considerably. 

 From the paroccipital process a strong ridge extends forward and 

 joins the petrosal ridge; while higher up, in front of the mastoid, the 

 base of the paroccipital process joins for a short distance the squamo- 

 sal. In front of each of the condyles is a large condylar foramen. 

 From the mesial border of one of these to that of the other, across the 

 basioccipital, is a distance of 60 mm. The width of the anterior end 

 of this bone is 47 mm. The bone is nearly flat on its lower surface. 

 In each upper outer angle of the exoccipital is a deep depression, quite 

 certainly a foramen; but the matrix has not been removed entirely. 

 The height of the rear of the skull from the upper lip of the foramen 

 magnum is 100 mm.; from the lower lip, 122 mm. 



The parietal (pis. 56, 58) is a large bone which has many of the 

 characteristics of that of Triclieclius, but it is more extended fore 

 and aft. Along the midline there are on the surface many fine 

 twisted lines and grooves which suggest that a median suture had 

 only recently closed. 



Weber ^ in speaking of the sirenians states that the parietals 

 are united in a sagittal suture, but in a skuU of Tnclieclms and 

 one of Halicore at hand there are seen no traces of this suture. 

 It is present in skulls of Hydrodamalis (RJiytina). On each side, 

 the parietal sends forward a narrow process which reaches nearly 

 to the rear of the orbit. In the wide notch between these is 

 received the hinder ends of the frontals. In both Trichechus and 

 Halicore the parietal sends down a process which joins the alisphenoid. 

 These bones join likewise in Eotlierium. In Desmostylus the two 

 bones are well separated by a process of the frontal, which extends 

 backward and joins the squamosal. On each side of the parietal, 

 about 20 mm. from the midline, is a low temporal ridge. From side 

 to side the parietal is arched, although slightly flattened between 

 the ridges just mentioned. 



Each squamosal is a large bone which contributes to the forma- 

 tion of the side walls of the brain-case, partly incloses the auditory 

 organ, forms a surface for the articulation of the lower jaw, and 

 sends a great process forward to assist in forming the zygomatic 

 arch. The suture formed with the parietal is 100 mm. long. The 

 lower hinder angle forms a rough post-tympanic process, which joins 

 the mastoid portion of the otic. The zygomatic process extends 

 forward to a point 160 mm. in front of the rear of the squamosal, 

 where it joins the malar. Unlike that of Tricliechus and Halicore, 



' Pie Siiugel.iero, p. 792. 



