31 



The mastoid (Plates XII. & XIII. fig. 1, s) forms a rugged process, in depth or length 

 not exceeding the paroccipital (4), but of greater breadth and thickness; above it, 

 externally, and probably in the line of the primitive suture with the squamosal, is a 

 venous foramen, Plate XII. fig. 1, v. The petromastoid — probably the petrosal part 

 in a greater degree — forms the hemispheric articular cavity (Plate XV. :a) for the 

 stylohyal (Plate I. ss), the anterior rugged wall of which cavity extends downwards 

 farther than any other part of the proper basis cranii, Plate XII. fig. 1, 16 : the petrosal, 

 anterior to this, sends down a shorter rough pyramidal process. The carotid 

 foramen (Plate XV. c), a full ellipse with diameters of 5 lines and 4 lines, is situated 

 between the petrosal and basisphenoid at the fore-part of that oblong depression 

 which is terminated behind by the large precondyloid foramen. 



The stylohyal (Plate I. 3 8 ) has the form of a hammer, with a long, slightly bent 

 handle, terminated by an obliquely truncated rough surface for syndesmosis with the 

 ceratohyal. At the opposite end the handle is subcom pressed, and the head is formed 

 by a sudden expansion in the vertical direction, terminated posteriorly by a straight 

 but rugged margin, and with the upper end produced, thickened, and forming a 

 smooth convexity, or condyle, adapted to the cavity above described in the petro- 

 mastoid. The lower end of the head or expanded part of the hammer-shaped bone 

 is more produced, more rugged, and terminates obtusely. The outer surface has a 

 wide depression at the middle, which is rough, with several short and well-marked 

 ridges. The length of the specimen described is 8 inches, the breadth or depth of 

 the expanded end is 3 inches and a half. 



The upper part of the coalesced frontals (Plate XIII. fig. 2, 11) forms a smooth tri- 

 angular plate, rapidly expanding to the postorbital processes (it) and very slightly 

 convex. Some indistinct traces of the fronto-nasal suture seem to show that the 

 nasal bones (Plate XIII. fig. 2, u) extended backward beyond the transverse parallel 

 of the postorbital processes : more distinct traces of the naso-maxillary sutures (21), 

 show that the coalesced nasals were 2 inches 9 lines across at their narrow posterior 

 part, where they are flat above : at first slightly contracting, they then gradually ex- 

 pand, and become more and more convex transversely to their anterior extremity. 

 Here the nasal bones are also thickened, are rugged for the firmer attachment of the 

 cartilaginous parts of the nose, and their under surface, being excavated by two longi- 

 tudinal grooves, the thickened terminal surface is divided into a middle (Plate XIV. 

 fig. 2, m) and two lateral («, n) parts, the latter being convex and subangular, and 

 the middle expansion slightly excavated. As in the Two-toed Sloth (Choloepus didac- 

 tylus), the under surface of each nasal bone sends off a terminal plate or process for 

 the attachment of a turbinal cartilage or ossicle. A narrow median groove indi- 

 cates the original suture between the nasal bones along their anterior half. 



The cranial cavity of the Megatherium is considerably smaller than the cranial 

 part of the skull, the outer wall or plate of bone being separated by large irregular 

 air-cells from the vitreous plate, or that case of bone which immediately invested the 



E 2 



