NO. 1540. THE SKULL OF BRACHAUCHENIUS—WLLLISTOK. 483 



open on the sides. It would seem that the chief support for the man- 

 dibular muscles must have been on the sides oi the parietals and the 

 stout postfrontals and postorbitals. 



The Uniits of the squamosal above can not be determined, owing 

 to the erosion of the specimen, as inchcated in the drawing. The 

 parieto-squamosal arch is, however, quite stout, narrowed anteropos- 

 teriorly near its upper part. The massive quadrates are exposed 

 below on the outer side and behind. Further information concerning 

 the occipital region can not be had until the matrix has been removed ; 

 the anterior cervical vertebras are crowded into this space. 



At the bottom of the large temporal vacuities the supraoccipitals, 

 exoccipitals, petrosals, and stapes were found more or less disarticu- 

 lated and separated. The larger part of the exoccipital is seen some- 

 what removed fi-om its relations to the stout supraoccipitals. Its 

 anterior, cranial surface presents a deep pit and marginal sutural sur- 

 faces, completed by imion with the supraoccipital and petrosal. The 

 paroccipital process is rather slender, directed downward, outward, 

 and backward in life, with its distal extremity flattened and appar- 

 ently spatulate, for union with the upper end of the quadrate, as 

 described in THnacromerum osborni. 



Petrosals. — The petrosal is a peculiar bone. That of the left side 

 has been wholly freed from its matrix; on the right side it lies with 

 its free, convex, outer side exposed near the front border of the tem- 

 poral vacuity. Exteriorly the bone is nearly evenly and smoothly con- 

 vex, shell-like. The iimer side I have figured in Plate XXXV (pet) , 

 natural size. Its precise mode of union with its two contiguous bones 

 can not be determined. Its two cHverging canals doubtless lead into 

 the supraoccipital and exoccipital sinuses or semicircular canals, as 

 I have found them in Trinacromerum osborni. The greater part of 

 the bone is deeply and smoothly excavated for the internal ear, leav- 

 ing a free border for the petrosal part of the large foramen ovale. 

 The excavation is deep and large for the size of the skull, much larger 

 proportionally than in the mosasaurs. 



Stapes if). — A small and peculiar bone, lying apparently nearly in 

 position in the matrix on the right side, I can determine only as a 

 stapes, a bone hitherto unknown among the plesiosaurs. It is a short, 

 stout bone, a side view of wliich is shown in Plate XXXV {st) nat- 

 ural size, not unlike a human metatarsal, though less slender, with 

 an attenuated, cyhndrical shaft, and an articular expansion at either 

 end. TVliat I believe to be its proximal end, from its position in the 

 matrix, presents a hemispherical articular surface, bounded by a 

 shelf -like ridge, as though for articulation in a foramen. The other 

 extremity is obliquely expanded, concave somewhat from side to 

 side, smooth, and with a partial longitudinal ridge near one side. 

 The extremity of the bone has been broken away. 



