62 
1SIJR. W. K. PARKER ON THE STRUCTURE AND 
binds down upon the fore part of the squamosal, and its oval hole (V 3 .) can be seen,, 
where the pterygoid also underlies the alisphenoid. Between the palatines and 
pterygoids the basal parts of the two sphenoids are shown. At their base the orbito- 
sphenoids (o s.) are fast meeting each other, presenting each to each a convex edge ; 
there is no bony presphenoid. The interspace of cartilage here enlarges into a 
roundish tract, bounded, behind, by the basisphenoid ( b,s .), the descending ribbed 
margins of which touch the orbitosphenoidal bones by their fore ends. The suture 
between the alisphenoids and their basal piece can be seen outside these crests. 
From the presphenoidal cartilage ( p.s .) to the foramen magnum (fm.) the basis 
cranii is scooped, or concave, and widens, gradually, to the end. The basisphenoid 
( b.s.) is one-third longer than wide, and is notched in a rounded manner at both ends. 
The cartilage between it and the basioccipital (&,<?,) is a transversely oval tract, and 
the two bones nearly touch by their outer margins. The rudimentary crests growing 
down from the edges of the basisphenoid are the homologues of the wings, so familiar to 
us in the Insectivora, that assist in the formation of the ear-drum. The much larger, 
thick, diverging ridges that grow from the edges of the basioccipital, are the exact 
counterparts of the ridges to which the elongated pterygoids are attached in the 
Ant-eaters (see in Cycloturus, Plate 10). Here the shortness of the palatines, and the 
arrest of the dilated pterygoids in front of the basioccipital, makes all the real 
difference there is in these parts between this Sloth and an Ant-eater ( Myrmecophciga , 
Cycloturus ), These outgrowths of the basioccipital of the Unau might have served 
the same purpose as the tympannic wings of the basioccipital in the Insectivora, or 
the alisphenoidal outgrowths in the Marsupial.* Here they serve no auditory purpose, 
but are used as strong points of attachment to muscular bands. Narrowing, again, 
from the ends of these ribbed edges, the basioccipital runs now nearly to the convex 
condyles (oc.c.), and has a deeply emarginate hind margin against the foramen 
magnum (fm.). In front of the reniform condyle there is an oval mass of carti¬ 
lage, and beyond it the exoccipital, pierced by the 12th nerve (XII.), and forming a 
low paroccipital ridge ( p.oc .) against the auditory capsule, As the occipital plane 
leans forward, above, the supraoccipital ( s.o.) is only seen at its lower edge, bounding 
the foramen magnum, behind. Bight and left of the basioccipital the auditory capsule 
is seen here and there, hidden largely by the drum-ring and membrane. Here the 
ossified manubrium (m.ml.) is seen, and opposite it is found the long-lipped Eustachian 
opening (eu.). The opisothotic region (op.) is seen outside the paroccipital, and 
running from, and through it, the epihyal, and facial nerve (e.hy ., VII.). 
The upper view (Plate 9, fig. 2) shows a strong, smooth, oval roof, composed mainly 
of three pairs of bones that lessen, forwards : these are the nasals, frontals, and 
parietals (n.,f.,p.). Together, the nasals have somewhat of an hour-glass shape, but 
they are pointed in front and spread out behind, at their frontal suture. The 
* The root of the “tympanic wing” in the Marsupial is a direct outgrowth of the alisphenoid ; the 
hinder part is the “os bullae,” anclrylosed to that root. 
