66 MR. W. K. PARKER ON THE STRUCTURE AND 
splintery, and hollow on the inside (fig. 6). Over this plate the head of the malleus 
grows like the hood of the Monkshood flower, a remarkable semioval growth of 
bone. The sinuous, saddle-shaped condyloid facets of the malleus and incus form, as 
the figures show, a very strong joint, and the latter bone is equal in size to the head 
of the former. 
Both the crura of the incus are short, but well formed, the short crus (s.c.7.) has an 
oval facet for articulation with the cupped space in the tegmen tympani, and the 
long crus (/.¢.7.) has a nearly circular and flat facet for the head of the stapes (st.). 
This latter bone is shown 7n sitw from the outside (fig. 7), and, detached and dislo- 
cated, from the inside (fig. 6). The little nucleus of cartilage representing the inter- 
hyal (7.hy., see fig. 9) is now a small spur on the neck of the short columella ; now 
by absorption of the centre of the flat stem, a veritable stapes. Here again we are on 
the Metatherian level; the Marsupials have a “ pharyngohyal” which, in ditferent 
types, oscillates between the columella and the stapes (see Doran, ‘ Ossicula Auditits,’ 
plate 64). The secondary perforation of the “columella,” making it into a sort of 
stapes, is seen also in Birds (see Doran, plate 64, figs. 44-46). In this figure 
(fig. 7), little except the cochlear part of the auditory capsule is drawn, with the 
fenestra ovalis (fs.0.) occupied by the stapes, and the fenestra rotunda (f7.). This well 
ossified mass must be considered as a thick section, cut off from the part that contains 
the canals, and from the opisthotie angle, behind (figs. 1, 3, 4, op.). 
T am satisfied that in the Sloths the periotic bony deposit is generalized, as in 
the Armadillos—and in certain anurous Amphibia. 
The epihyal (fig. 8, e.hy) is still cartilaginous, and is followed by a single ossified 
ceratohyal segment (c.hy.), and then by an unusually long hypohyal (h.hy.), now 
largely ossified. The U-shaped basal piece (b.4.br.) is also getting a bony centre in 
each thyrohyal horn (t.hy). 
Skull of half-grown Common Three-toed Sloth (Bradypus tridactylus, Linn.).—Fourth 
Stage. 
A dry skull of the half-grown young of this species has served to make many things 
plain to me in this type. 
The side view, with the olfactory, auditory, and most of the occipital, region 
removed (Plate 13, fig. 5), shows the roof bones (x., f, p.), the nasals, frontals, and 
parietals, supported behind by the large supraoccipital (s.o.), and below by the 
sphenoids (V!. to V°.) and the superficial squamosal (sq.). The nasals (7.) are short, 
the frontals (f) have but an indefinite supraorbital rim, and the parietals have a 
sinuously swelling general surface. The squamosals (sq.) are less produced forwards 
and backwards than in the Unau (Plate 9, fig. 3, sq.), but have a more well-marked 
postglenoid process ; the zyomatic process also is longer. 
The thick short palatine ( pa.) and the long swollen pterygoid (pg.) are seen in 
