576 
ME. W. K. PAEKER ON THE STRUCTURE AND DEVELOPMENT 
In the presence of such curious facts as the skulls of larval Caducibranchs disclose, 
the mind naturally seeks to know what have been the factors in that marvellous modifi- 
cation of the axis seen in the skull of a Vertebrate animal. I shall reconsider these 
things in my “ Summary.” 
The auditory capsules of the larval Seironota (Plate 29. figs. 1, 2) are thoroughly 
chon drifted, and well show the form of the enclosed membranous labyrinth, with its 
three canals above (fig. 1, a.s.c., h.s.c., p.s.c.), and the crescentic slit, becoming the 
fenestra ovalis (fig. 2 helow. 
Mesiad of this oblique rent in the capsule the ragged and imperfect cartilage has 
not yet formed itself into a stapes. 
The suspensorium has all its three upper processes formed ; and the ascending (fig. 1 , 
a.p.) is grafting its apex on to the alisphenoidal crest. The pedicle (fig. 2, pd.) is blunt 
and rounded ; a considerable space separates it from the trabecular convexity. 
The otic process ( ot.p .) is equally well seen in both aspects ; but above (fig. 1) it 
sends forwards and inwards a pedate lobe that cleaves close to the ear-sac against the 
anterior ampulla, its normal terminus. 
The lower surface is gently convex; but the upper is hollowed, relatively, to the 
ascent of the otic and ascending processes. The direction of the clubbed quadrate 
region is forwards, and so far outwards, that a line, parallel with the axis of the skull, 
which should pass through the otic process behind, would cut the inner face of the 
condyle in front. 
Both the articular faces are somewhat convex and then sloping, the movement of the 
articular end of the Meckel’s cartilage (mlc.) on the quadrate being loose and free, like 
a Cow’s jaw. 
The two cartilaginous mandibles make the face somewhat underhung ; together they 
form half an ellipse, and become small, by degrees, to the chin, where they are united 
by fibrous tissue. 
In this early state we miss the pterygoid process of the suspensorium, and the etlimo- 
palatine visceral rudiment. 
Bony laminae are fast appearing over this simple chondrocranium. Above (fig. 1, 
f-,p-, sq.,px., d., sp .) Ave see these films, that already have taken on the outline and 
form of the frontals, parietals, squamosals, premaxillaries, dentaries, and the denti- 
gerous splenials. 
The articulars, nasals, ethmoids (outer and inner), the exoccipitals and the prootics, 
none of these have appeared. 
Beneath (fig. 2, pa.s.), the large parasphenoid is flooring the unfinished skull with 
its open rafters (tr .) ; and in the olfactory region there is a triangular tract of teeth on 
higher Vertebrata. I have long ago shown that the notochord of the early chick is submoniliform (“ Eowl’s 
Skull,” plate 82. fig. 3) ; and it is not an unscientific use of the imagination to suppose that the Sauropsida 
and the Mammalia have a series of three or four, or even more vertebrae suppressed in the region of the cranial 
notochord. 
