DEVELOPMENT OF THE SKULL IN THE URODELES. 189 
gently convex, smooth, and of moderate thickness; it reaches nearly to the foramen 
magnum (f.m), is rounded, leaves the occipitals uncovered, and widens in a leafy, 
lobate manner up to the fore edge of the auditory capsule. Its margin then is cut 
away in a concave manner, and is less by one third; in the orbital region it completely 
floors the endocranium, enlarging gently up to the aliethmoid (t). The bone then 
loses another third of its (greatest) breadth, lessens still more forwards, and then ends 
in a rounded manner close behind the decurved processes of the premaxillaries. The 
narrowing of the parasphenoid in front is sufficient to show the edge of the premax- 
illaries in the under view (Pl. XX XVIII. fig. 6, pa, pa.s). 
The next bony plates on the inferior face of the skull are the vomers (Pl. XX XVIII. 
fig. 6, and Pl. XX XIX. fig. 1, v), to which are attached the arrested palatines (pa). 
These two pairs of bones are in a very rudimentary condition, being an oblique bony 
tract uniting into one series the anterior palatine teeth, and having the hinder fourth 
segmented off as a palatine rudiment, just as in the embryos of the highest Urodeles 
or “ Caducibranchiata” (see Pl. XL. fig, 1, v, pa). 
These vomero-palatine tracts are arranged as a loop or arch, unfinished in front at 
its crown, the two tracts being united by a short ligament; the fore end is narrow, and 
the hinder end broad; and each tract is sigmoid, and is covered beneath by ten or 
twelve sigmoid rows of long, sharp, recurved teeth (Pl. XXXVIII. fig. 6, and 
Pl. XXXIX. fig. 1). 
About three of these rows belong to the palatines (behind), and the rest to the vomers ; 
where the palatines and vomers unite there is the inner outline of the internal nostril; 
this dentigerous structure is extremely elegant on account of the regularity and curve 
of the rows. The paired bones are curved in a sigmoid manner, so that each row repeats 
the same curve on a smaller scale; it has all the beauty of a doubly compound leaf’. 
The investing bones of the lower jaw are remarkable, the dentary (d) forming half 
the mass of the jaw, and covered with horn at its dentary edge, whilst the splenial, 
which carries teeth, is very small. 
Like the upper jaw the lower jaw is much decurved; it is very large and massive. 
The dentary (Pl. XX XIX. figs. 1, 3, d) is very thick and solid where it meets its fellow, 
to which it is joined by strong ligamentous fibres; it becomes double its front height in 
the middle; and this “coronoid” crest is so far forward as to be below,the ethmo- 
palatine cartilage (e.pq). 
1 The Siren is very instructive in several respects; but the fact of its having the outer dental line covered 
with horn, whilst the inner only carries vomero-palatine, and, as we shall see, splenial teeth, is like a mixture 
of the Tadpole and the adult Frog in one type. It may be noted that the inner or submarginal line, in cases 
where there are two or more rows of teeth, as in Lepidosteus, may carry the largest of those organs. As 
an exceptional type in its own group it may be compared to the larvee of Dactylethra and Pipa, where, contrary 
to the rule of their Order, there is no suctorial mouth covered with horn. 
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