DEVELOPMENT OF THE SKULL IN THE BATRACHIA. 105 



24 (continued). — (B) Camariolkis tasmaniensis (?). — Larva, f inch (9 lines) long ; legs, 

 -g^Q inch. Same locality. 



This Tadpole was the largest of several of this species examined by me, and its free 

 hind legs shoveed that it has attained to its full larval condition ; it was as long with 

 its tail as its parent without. 



In the possession of paired nasal sacs, each with a distinct roof, and of a well defined 

 fenestra ovalis, this larva had gone beyond the Lamprey ; but there was no bony 

 deposit even below the skull (parasphenoid), and the hinder half of the long hypo- 

 physial space was still membranous ; the rest of the space was composed of a very 

 thin layer of young, half consistent cartilage. 



This skull, therefore, was in the best possible state for comparison with that of 

 Fetromyzon and its allies. 



Leaving out the prenasal structures this skull (Plate 15, iigs. 6, 7) is nearly square ; 

 it is unusually short, and although the head was no larger than that of a Blow-Jly, the 

 chondrocranium had become a very solid structure ; and cartilage was forming even in 

 Tadpoles one-third the length of this — that is, in newly-hatched specimens corres- 

 ponding to my earliest stage in Bn/o vuhjaris (Phil. Trans. 1876, Plate 55, fig. 1). The 

 fii'st-formed cartilage can now be well seen, as it is much more massive, and richer 

 with proliferating cells, than the newer tracts. 



The cartilage, which at first only enclosed the apex of the notochord as the ends of the 

 trabeculse, has now spread along the whole hind floor and a very definite tract crosses in 

 front of the notochord. That rod is expanding to its full (spinal) size just where it 

 emerges from between the parachoixlal bands(ii'.); they pass outwards and backwai'ds, and 

 end in a free rounded point behind the ganglion of the 9th and 10th nerves (IX., X.), 

 which lies in a notch in the outer border of each band. These bands are distinct 

 until they reach the apex of the notochord, and then, as just mentioned, unite in front 

 of it ; there they form the hinder boundary of the pituitary space. Directly in front 

 of the notch for the ganglion each parachordal dilates suddenly into a bx'oad crescentic 

 wing, which forms a concave floor for the antero-internal half of the auditory capsule 

 (fig. 7) right and left. There is a shallow crescentic notch in the middle of the hinder 

 margin of each wing ; this fonns the inner boundary of the fenestra ovalis (J-O.). In 

 front, there is a deep round notch between the wing and the trabeculse ; in and over 

 this space lie the more or less united ganglia of the 5th and 7t]i nerves (V., VII.). 



The sub-orbital fenestra, and the band of cai'tilage which encloses it are, together, 

 scaixely of larger extent than the auditoiy capsules behind. These have lost their 

 simple oval form, for the three large canals and their globular ampulla? {a.sx., h.s.c, 

 p.s.c.) above, and the sacculus (rb.) below, have wrought the sac into their own form. 



The alate basal floor is thin, and composed of rather young cartilage, but the 

 auditory sacs are very solid, for they become cartilaginous directly after hatching, 

 coevally with the trabecular and suspensoria. 



MDCCCLXXXI. p 



