DEVELOPMENT OP THE SKULL IN THE BATRACHIA. 123 



entirely roofed over l)y tliem ; and the orbital spaces, so large generally, have only 

 one-third the extent above that they possess below. 



The endocraniurn is not intensely ossified, and there is no increase of the number 

 of bony centres either in it or in the external plates such as is seen in Rana inpiens ; 

 but it agrees with that large Frog in the moderate ossification of the inner skull, and 

 in the extension backwards of the condyles for the lower jaw. 



But on the whole, leaving out of view the dense overgrowth of the subcutaneous 

 bones, this skull differs less from that of Rana tigrina and R. hexadactyla (Plates 

 6 and 7) than it does fi'om the skull of R. 2^ipiens. 



It agi'ees with that of the Oriental Bull-frogs in the form of the endocraniurn ; in 

 the relative size of the cranial trough (much wider and larger than in R. 2nx>iens), and 

 in the severely typical paucity of bony centres, whether external or internal. 



On the whole, there is little in this skull of divergence from the " norma," except 

 the exuberant growth of its roofing plates ; and in this genus, and also in Ceratophrys, 

 it is in this overgrowth, and not in indefinitely numerous bony patches, that we have 

 the mark of ancient ness : the great size of the species, however, is in itself suspicious in 

 a group whose members are for the most part small, and even very small. 



The occipital condyles (Plate 21, figs. 1, 2; and Plate 22, fig. 1, oc.c.) are large, 

 oval, postero-inferior in aspect, and separated by the basioccipital cartilage, which is 

 gently emarginate, and one -third of the width of one condyle. 



The superoccipital ai'ch (Plate 22, fig. 1) is excavated above, and less than a line 

 in front of the basal outline (Plate 21, fig. l,f.m.); it is not finished with bone, for 

 there is a considerable wedge-shaped tract of superoccipital cartilage. 



Both the auditory and ethmoidal I'egions stand out at right angles to the axis, and 

 the former is twice the extent of the latter. The interorbital region between these is 

 of the average length, and twice as broad as in Rana pipiens (Plate 8) ; it narrows 

 gently up to the ethmoidal region, and is almost straight-sided, not bulging, as in many 

 types. The depth (fig. 3) is proportional to the width, and the sectional form is boat- 

 like, gently widening upwards. The foramen ovale is partly rimmed in front by the 

 prootic (fig. 2, v., pr.o.) ; the optic fenestra (II.) is of the medium size and is midway 

 in a tract of cartilage that occupies half the interorbital region. The girdle-bone {etli.) 

 takes up the rest ; it spreads some way into the ethmoidal wings, but does not reach 

 quite to the front of its own proper territory, nor ossify the limited superorbital 

 lamina (s.oh.). 



Behind, the ex-occipitals and prootics (fig. 2, e.o., pr.o.) are separated by a triangular 

 tract of cartilage, and do not extend outwards as far as the jDarasj^henoidal wings ; 

 thus the lesser vestibular and the larger tegminal tracts (fig. 2) are left soft. 



The tegmen tympani {t.ty.) is largely extended outwards, beneath the over-arching 

 squamosal (sq.), as a flat, sub-quadrate, pterotic tract ; the vestibular floor is gently 

 convex, and is confluent on its outside with the dilated stylo-hyal end of the hyoid 

 bar (st.h.). 



