DEVELOPMENT OF THE SKULL IN THE BATRACHIA. 83 



17 (continued). — (E) Skull of adult Pseudis paradoxa. — Old male; 2\ inches long. 

 Surinam. (Hyrtl's prepn. in Mus. Coll. Surg., Eng.) 



The specimen whose skull is here figured was evidently a very old individual ; it is 

 therefore of great value in this series.''^ 



It is at once seen to be a very generalised type (Plate 10, figs. 1-4), sharply 

 separated from a normal Frog, such as Rana temporaria. 



The skull is one-half the length and a little more than half the breadth of that of 

 the perfect larva. 



The skull of the nearly metamorphosed larva (D) is five-sixth a the length of that of 

 the adult : so that after lessening much, it enlarges again, a little. 



About the size of the skull of a large Common Frog, it is more elegant in shape, 

 being a very perfect semi -ellipse in outline. 



The well-bent bow formed by the two series of cheek bones has its " horns" meeting 

 at a sharpish angle, for the parts in front are modelled on three trabecular outgrowths, 

 the " pro-rhinals " and a small "prenasal." 



The complex structures that run across, behind, from one cheek to the other, are 

 also very beautiful in their construction (Plate 10, figs. 1-3). 



The hind part of the skull is densely ossified from the occipital condyles to the optic 

 foramen (oc.c, II.), and in front the ethmoidal girdle-bone {eth.) occupies the hind half 

 of the nasal capsule below, and all but the edge in fi'ont ; the parts in front of that, 

 and the anterior sphenoid (o.s.), are unossified. 



The occipital condyles are rather wide apart, being separated by a large semicircular 

 emargination ; they are hemispherical and subpedunculate, as in those Urodeles that 

 have an intercalary " odontoid " vertebra. 



Below (fig. 2), the auditory masses seem to stand out in a directly tran.sverse line ; 

 but above (fig. 1), they are seen to turn forwards as well as outwai'ds, and the well- 

 marked semicircular canals (between pr.o. and e.o.) are some distance from the edge, 

 which is flanked by the squamosal (sq.) : the intervening part is the ossified " tegmen 

 tympani." 



The gently rounded floor of the vestibule (fig. 2) shows no trace now of that 

 Selachian development of the basal plate, outside the capsules, which formed the 

 tymj)anic floor. 



The main nerve-passages (fig. 2 ; II., V., IX., X.) are well seen from below ; the two 

 hinder passages are separated by a bony bar ; the " foramen ovale " is enclosed in bone, 

 and the optic fenestra is partly margined, also, by the extended alisphenoidal wing of 

 the prootic {pr.o.). 



The foramen magnum has less roof than floor ; neither of these plates ha.s any 

 divisional line as in a i^ormal Batrachian. 



• It is a mo.st fortunate thing for mc that our College has possessed itself of this invaluable specimen ; 

 it is not an easy thing to get au adult. 



M 2 



