560 , PEOE. OWEN on teoeastomus sieenoides. 



figs. 3, 5 ) persists, as shown by Yrolik in his subject of Manatus 

 americanus *, and by Home in the skull of Halicore mdlayana f. The 

 longitudinal contour of the basioccipital is wavy, concave below at the 

 hinder half, convex in fronts decreasing in breadth as it approaches 

 the basisphenoid. On each side of the basioccipital is a large vacuity, 

 in which appears mesially the petrosal (Ib. ib. ie) and laterally the 

 tympanic. The lower (exposed) surface of the -latter shows the 

 Sirenoid thickness and convexity. 



The basisphenoid expands a little beyond the suture, and the sides 

 bend down, forming a broad and deep arch or concavity downward, 

 as in Manatus ; but there is no trace of the pair of tuberosities on 

 the basilar tract as in Manatus, Halicore, and BJiytina. 



In advance of the paroccipital (4) is the expanded base ( 8') of a 

 vertical mastoid ridge (PL XXYIII. fig. 1, 4 ) extending from the 

 hind part of the squamosal (27), between which ridge and the 

 origin of the zygoma (27') is the upper and hinder arch of the tym- 

 panic cavity (t). The zygomatic process of the squamosal is verti- 

 cally broad or deep ; its upper border rises and describes a bold 

 convexity ; its outer surface is undulated, convex at the upper half, 

 concave vertically below. "What appears to be the hind end of the 

 malar part of the zygoma (PL XXYIII. fig. 1, 26') has been dis- 

 placed from its suture with the squamosal portion (ib. 27'). It is 

 of equal or of greater vertical extent, and must have given the 

 zygomatic arch characteristically Sirenoid proportions. The origin 

 of the zygomatic process of the squamosal does not project abruptly 

 backward or upward as in Halicore and Felsinotherium ; it is rather 

 nearer to Manatus in this respect, but of more normal character 

 than in any other Sirenian. The squamoso-parietal suture does not 

 rise above the level of the upper border of the zygoma ; it extends 

 backward to the exoccipito-mastoid ridge, with an angular depres- 

 sion at the middle of its course more open than in Manatus. 



The upper wall or roof of the cranial cavity is broken away ; but 

 the ridge curving from the frontal to the postorbital angle remains 

 on the left side. The breadth of the brain-case (2j inches in ad- 

 vance of the foramen magnum) is 2 inches, indicating the Sirenian 

 retention of the breadth of the cerebrum to near the fore end of that 

 part of the brain. In advance of this the cranium contracts to a 

 breadth of 1 J inch, which is in a greater degree than in other known 

 fossil Sirenian s, or in any of the recent forms. The superorbital plate 

 of the frontal (ib. fig. 1, 12) is of great vertical thickness, and projects 

 with a convex border above the orbit (0). This is elliptic, with the 

 long axis parallel with that of the skull. The lower border is 

 formed by the malar (26), which, seemingly entire on the right side, 

 projects a little beyond the upper border of the cavity. The fore 

 part of the orbit is bounded by a thick outstanding subvertical plate of 

 the maxillary (21), with an indication of a small imperforate lacrymal 

 (73) between it and the frontal. The wide suborbital canal (21') opens 



* Bijdragen tot de natuur- en ontleedkundige Kennis van den Manatus ameri- 



nus, fol. pi. iv. fig. 12. 



t Phil. Trans. 1820, pi. xii. fig. 1. 



