DEVELOPMENT OF THE SKULL IN THE UKODELES. 197 



The Skull of the Larva of Triton cristatus, 2-^ inches long, with branchiae very large. 



A. The Investing Bones. 



The larval Newt is now truly representative of the " Proteidea." Its skull is not quite 

 like that of any of the large " Perennibranchs," but is similar to and illustrates any or 

 all of them. 



The parietals (PI. XL. fig. 2, p) are well developed ; they reach back over the occipital 

 roof [oc. r) to the middle of the auditory capsule, and stretch forwards, as narrow wall- 

 plates, to the ethmoidal region ; laterally each bone projects over the temporal region 

 by a concave rounded process, which almost touches the squamosal — its normal supero- 

 lateral correlate throughout the vertebrate series. 



Narrowing in front of the scooped temporal wings, the parietals run forwards, getting 

 less and less, and allowing the frontals to imbricate them in front and at the sides. 

 The latter bones [f) together form an oval tract, narrow behind and broad in front ; they 

 do not reach the edge of the cranial wall, and end on the ethmoidal region, imbri- 

 cated by the nasals and nasal processes of the premaxillary {n, n. px). 



The nasals (w) lie outside those processes {71. pi'), and are long, thin, falcate bones, 

 with their convexities inwards ; they reach nearly to the front, and curve outwards 

 behind, following the nasal roof (na). 



The precocious premaxillary (px) is now a large bone, whose dentigerous margin 

 (fig. 3) is a quadrant ; the nasal processes, like the nasal bones, are long and falcate, 

 and have their convexity inwards. 



The diverging splintery hinder parts of these two processes are united by an 

 isthmus of bone ; in front of that bar there is an hourglass-shaped chink ; the enlarge- 

 ment at the fore end is due to the presence of the " middle nasal passage " {m. n.p) : see 

 also fig. 3, where it is seen as an oval space in the palatine processes of the bone. 



The maxillaries and prefrontals have not appeared as yet. The squamosals (sq) are 

 hammer-shaped splints ; the cochleate head is applied to the ear-capsule, and the styli- 

 form handle to the postero-external edge of the suspensorium. 



Below (fig. 3) the vomero-palatine tracts of bone have au incurved, hooked blade, 

 and a flat dilating handle : most of the blade belongs to the vomer (v), now quite 

 distinct; the rest is the palatine root of the palato-pterygoid bone (p-pg). These 

 anterior parts are covered with recurved teeth. The shaft is narrowish at first, and 

 then becomes subflabelliform, forming a rest for the fore edge of the suspensorium. 



The parasphenoid (^a.s) is a large suboblong plate of bone, with gently concave 

 upper, and convex lower surface ; it is rounded behind, near the foramen magnum, 

 angulate in the preauditory region, and has its sides first enlarging and then lessening 

 just a little ; but it is broad, and has a crescentic emargination in front, parallel with 

 the internasal emargination, but ending at the hinder third of that tract. 



2n2 



