252 MR. W. K. PARKER OX THE STRUCTURE AND 



This archaic, generahsed skull helps us to see how severe, " in number, weight, and 

 measure," the morphological law is, that has, at last, reduced the Anurous type of skull 

 to the elected simplicity of that of the Common Frog. * 



SUMMARY. 

 A. — Primitive form of chondrocranium. 



In a Common Frog or Toad, soon after hatching, whilst the true oxter (or cutaneous) 

 gills are present, cartilage appears in the cephalic region. 



This first endoskeletal framework consists of three sub-parallel bands on each side, 

 that converge a little forwards, are some distance apart, and are almost entirely in 

 front of the notochord (Phil. Trans., 1876, Plate 55, figs. 1, 2). 



Other cartilages form about the same time, immediately under the skin, as labials in 

 front, and as branchial pouches beliind ; but these I shall not now describe ; only the 

 true endocranial elements. 



The innermost pair of bands, together, form a lyriform stnicture ; they are the 

 largest ; by their hind part they embrace the notochord at its apex ; they diverge 

 suddenly, enclose a large pyriform space under the fore-brain, converge nearly to 

 touching, and then diverge again, as short, broad, decurved horns. 



These are the " tralieculte cranii ;" they are /:)rt?-rt-chordal, behind, and the rest of 

 each bar is j:»-o-cliordal : there is no other parachordal cartilage, as yet ; the huge 

 notochord, which only gradually lessens to its rounded end, has, right and left, two 

 jiairs of " muscle-plates " enclosing it in the cranial i-egion. 



Where the trabeculse are most bent, behind and in front of the eye-ball, there the 

 second band is contmuous with the trabeculse ; by this double conjugation it encloses 

 an oval (sub-ocular) space. 



In front, each of these outer bands turns inwards towards the horns of the trabeculae, 

 and develops an oval, short, segment, which also turns inwards. This twice-conjugated, 

 second bar, is the " suspensorium " of the mandible ; the short segment is the mandible 

 itself, or the articulo-MeckeUan rod. The hinder conjugating pai-t is the "pedicle," and 

 the fore band is the pterygo-palatine rudiment. 



A little behind that rudiment, on its under face, the suspensorium has a broad 

 lozenge-shaped cartilage articulated with it — the third band ; this is only half as long, 

 but twice as broad as the second, and is the lower half of the hyoid arch — the cerato- 

 hyal : the upper half, or "epi-hyal," is not developed until two or three months after 

 the transformation of the Tadpole. Opposite the junction of each first and second 

 muscle-plate, there is a hollow ball of cartilage, unfinished (membranous) above ; these 



* For the description of the skulls of the " Aglossa" — Nos. 74 and 75 — the reader is referred fo my 

 former paper (Phil. Trans., 1876, Plates 56-62, pp. 62.5-66.5) ; and "Summary," infra, p. 255. 



