DEVELOPMENT OE THE SKULL IN THE BATKACHIA. 
657 
bone the cartilage escaping from the sheath is at first thick, and it bends downwards 
and inwards before it spreads into the extrastapedial ( e.st .), which is like a water-lily 
leaf, as in Dactjlethra. 
In the interim between this and my first stage I have missed the formation of the 
U-shaped “ annulus ” ( a.t .) on which the lips of the extrastapedial rest, but I have 
traced its formation in several kinds. 
The rest of the auditory capsule may be described here. 
The extreme degree of ossification of the whole “ occipito-otic ” region satisfies me 
that here, as in Dactylethra and Pseudis, it would at any time have been impossible to 
find any gap between the exoccipital and the prootic. These parts are now like a 
hard double fruit, with as hard a connective between them ; this intermediate part 
is all strong, save at the mid line below (Plate 61. fig. 3, b.o., e.o .). On the. lower 
surface of the hard ear-ball there is also a break in the bony covering, like a line of 
dehiscence running in from the middle of the outer edge. This chink in the hard floor 
is, however, due to the manner in which the bony growth has radiated ; it marks out 
the prootic region in front from the opisthotic behind ; but it does not, even in the 
adult (Plate 62. fig. 9), run round behind the fenestra ovalis to form the well-known 
opisthotic hook, as in the Common Toad (Plate 54. fig. 7, op.). This common bony floor 
grows, shell-like, to the front edge of the capsule, but leaves an external lip of cartilage, 
and fails to floor it in towards the inner edge also. Where it curves and narrows inwards, 
running round and between the ninth and tenth nerves (9, 10), it floors three fourths of 
its own side towards the basioccipital synchondrosis, with the overlying notochord ( n.c .). 
The bone runs to the lower edge of the foramen magnum (f.m.), leaving the middle 
and the condyle (oc.c.) soft. That condyle shows most above (fig. 2), and its face looks 
upwards and outwards ; its shape is oval, and it is unusually flat. 
Embracing these facets, the bone then wholly enarches the foramen magnum and the 
superoccipital region ( s.o .) ; but beneath the bone the cartilage is perfect (Plate 60. 
fig. 5, s.o.). The whole oval top, which is larger than the floor, is roofed with con- 
tinuous bone, which has set up absorption in the cartilage beneath (Plate 60. fig. 5), 
but not to the same extent as in the lower face. Through the strong but thin ectosteal 
plate and cartilage the semicircular canals (they are represented by dotted lines) can 
be seen in a well-prepared skull, examined as a transparent object*. 
The cartilage reappears in the alisphenoidal region in front, and in the tegmen out- 
side (figs. 2 & 6). The chondrocranium has other ectosteal plates that properly belong 
to the endoskeleton. One of these can be detected binding together the frontals 
(fig. 2,/.), which has, however, but little independence as a bone ; this is the super- 
ethmoidal plate (figs. 2 & 4, s.eth.), a large and permanently distinct bone in Dactylethra 
(Plate 59. fig. 1), but here already a mere spike and under-thickening to the middle of the 
fore edge of the common frontal plate (/.). The pterygoid bone (Plate 61. fig. 3, pg.) 
* After careful dissection, these skulls are treated with an ammoniacal solution of carmine, and then placed 
in glycerine. 
MDCCCLXXVI. 4 Y 
