DEVELOPMENT OF THE SKULL IN THE BATRACHIA. KW 



the Nearctic and Neotropic regions, tliut the Indian " Polypedatidas " do to the typical 

 Frogs of theii' own region. 



In general development the skull is about equal to that of I'oli/pedates vuiculaius 

 and L-hloronotus (Plates 27 and 26), but the ear-organs are more arrested ; altogether, 

 however, it is a lighter and more elegant skull, and is almost exactly intermediate 

 between the skull of a Ci/stignathus (of the same size), on one hand, and that of such a 

 true Hyla as H. ruhra (Neotropical), (Plate 33, figs. 6, 7) on the other. I shall com- 

 pare it with that of Cijstignathus ocellatus and typhonms (Plate 16). 



The length is nine-tenths of the gi'eatest breadth (Plate 29, figs. 7, 8), and the cheeks 

 rapidly narrow in towards the broad, rounded snout. 



The fore and middle regions are of equal axial length ; the liind skull is only 

 two-thirds their length, but it is of great breadth; the ossification, outer and inner, is 

 typical ; the whole structure is elegantly light, but of considerable strength. The 

 occipital condyles (oc.c.) are small, project but little, are directly posterior, and are nearly 

 twice theii" own breadth apart ; they are separated by a gently emarginate line ; the 

 condyles for the mandibles reach as far back as the fore edge of the stapes. The upper 

 and lower median cartilaginous tracts are rather wide, and the superoccipital edge 

 (fig. 7,f.m.) is almost as far back as the basioccipital. The prootics and ex-occipitals 

 {pr.o., e.o.) of the same side are continuous, and the bone reaches in front to the optic 

 fenestra (II). 



The bone, outside, reaches beyond the horizontal canal (h.s.c.) and then tliere is a 

 large lozenge-shaped tract of cartilage which ends in the tegmen tympani {t.ty ), whose 

 margin stretches outwai-ds and forwards. 



In front of the ampidlte of the anteiior and horizontal canals [a.s.c, h.s.c.) there 

 is a blunt hook of bone, looking outwards ; this process is also developed largely in 

 Siren lacertina (Huxley, Art. "Amphibia," Encyc. Brit., 9th edit., p. 758 ; and 

 WiEDERSHEiM, " Das Kopfskelet der Urodelen," plate 1, figs. 11, 12, Y. F., and 

 plate 2, fig. 18, V. F.). I have worked out this part in Siren, and in the correspond- 

 ing stage in the larvae of Triton cristatus, and I find it to be a foregrowth of the plaster 

 of cartilage developed beneath more or less of the auditory capside — an outgrowth 

 from the bevelled edge of the basal plate, or " investing mass." 



The hinder roof (tegmen cranii) reaches to the optic fenestrse ; it has two lesser 

 fontanelles in it. The main fontanelle is a long oval, half the extent of the roof 



The fontanelle is narrow because the tegminal edges are ■wide, equal to the scanty 

 tegmen in front — the top of the girdle-bone {etli.) ; tliere is a small open space there, 

 and a lozenge-shaped naked tract of the ethmoid. The temporal fossae are large and 

 round, being margmed outside by the prootic spurs ; then the rather nai'row skull 

 steadily widens forwards, as in Cystignatltus ocellatus (Plate 16, figs. 1, 2). 



Infero-laterally, two-fifths of the mid skull is unossified, the large optic fenestra} 

 lying at the end of the soft tract, right and left. 



In this flattish skull the girdle-bone barely reaches the true nasal region, and does 



MDCCCLXXXI. Y 



