DEVELOPMENT OP THE SKULL IN THE BATRACHIA. 163 



The thyro-hyals {t.hy.) are as long as the basal plate at its middle ; they are slender, 

 straight, moderately divergent, and well ossified. 



The investing bones are thoroughly Cystignatliine, and very elegant ; the fronto- 

 parietals (fig. 7, /.p.), are dilated in the temporal and ethmoidal regions, are hollowed 

 out to form the temporal fossa, and spread well over three-fourths of the hind skull, 

 where they end by a straight margin. They widen steadily in the orbital region, and 

 taken together, their fore-edge is crescentically emarginate— so much so that the great 

 fontanelle is not quite covered, and a large lozenge-shaped tract of the girdle-bone is 

 left naked. The diverging outer angle of each lies on the corresponding nasal. The 

 nasals {n. ) have the normal shape ; they are large, convex shells, and each touches the 

 septum nasi by its inner edge ; their bluntish fore ends leave the snout uncovered for 

 some distance. 



The marginal bones (j)^-, ''nx., q.j., sq.) are well developed and perfectly normal ; the 

 squamosal, however, only lies on the edge of the tegmen tympani ; its post-orbital 

 process is bent outwards, and is curved and long ; the descending part is normal. 



There is a very small, sesamoid septo- maxillary (fig. 7, between u.l~. and al.n.). 



The parasphenoid (fig. 8, 2^<^-^-) is long, well-developed, and normal ; the bone is 

 thickened at the cross, and the ti'ansverse jarocesses are carinate, the thickening 

 rmiuing along them as a ridge ; the " handle " is unusually long and slender. 



The vomei'S (fig. 8, v.) are very large triradiate plates; there is a posi-narial, but 

 not a /)re-narial spur ; and the front part is triangular, running under the edge of the 

 dilated subnasal lamina (right and left of s.n.) ; these, right and left, are far apart. 



The hinder half of each bone, on the contrary, converges towards its fellow, each 

 almost reaching the point of the parasphenoid. From the hind margin a tlnck rib 

 grows, covered with a rasp of retral teeth ; these lobes reach outw^ards further than 

 the inner edge of the inner nares {i.n.), their thick end is outside, and they are 

 furthest forward there. This hmd margin is shghtly arcuate, so that they run across 

 the fore palate as a crescentic rasp, just broken at the mid-Une ; they are quite 

 Cystignatliine. 



Most of the divergencies from the "norma" in the skull in this species are veiy 

 gentle, and yet quite appreciable ; this is a true Neotropical Frog, with the toes dilated. 



Its main points of difference are as follows : — 



1. The nasal region is extremely large, both wide and long ; and the orbital region 

 steadily widens towards it. 



2. The hind skull is unduly ossified. 



3. The prootic region has a projecting spike, as in Siren lacertina, and the lars-a of 

 Triton cristatus. 



4. The jDalato-suspensorial arch is very slender. 



5. The quadrate is largely ossified. 



6. The annulus is small and widely open. 



7. There is neither an inter- nor a s?<_pra-stapedial. 



Y 3 



