DEVELOPMENT OF THE SKULL m THE UEODELES. 189 



gently convex, smooth, and of moderate thickness ; it reaches nearly to the foramen 

 magnum (/. 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 (tr). The bone then 

 loses another third of its (gi-eatest) breadth, lessens still more forwards, and then ends 

 in a rounded manner close behind the decui-ved processes of the premaxillaries. The 

 narrowing of the parasphenoid in front is suflacient to show the edge of the premax- 

 illaries in the under view (PI. XXXVIII. fig. 6, px,pa.s). 



The next bony plates on the inferior face of the skull are the vomers (PI. XXXVIII. 

 fig. 6, and PI. XXXIX. fig. 1, ■;;), to which are attached the arrested palatines {im). 

 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 PI. 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, recuiTed teeth (PL XXXVIII. fig. 6, and 

 PI. 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 veiy large and massive. 

 The dentary (PI. XXXIX. 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.pa). 



' The Siren is very instructive in several respects ; but the fact of its having the ov,ter 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 tj-pe. It may be noted that the inner or submargiual Hue, 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 larvs of Daciylethra and Pjpa, where, contrary 

 to the rule of their Order, there is no suctorial mouth covered with horn. 



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