DEVELOPMENT OF THE SKULL IN THE URODELES. 187 
14. There is a rudimentary ceratobranchial on the third arch; both parts are ossified 
in the second branchial; and all the epibranchials are more or less ossified. 
15. The hind part of the second basibranchial is nearly all absorbed, distal rudiments 
only remaining as thyrohyals ; the first basibranchial is a very large foliaceous cartilage, 
and is not distinct from the rudiment of the second. 
16. There is a prenasal bone. 
E. Comparison of the Skull of Menopoma with that of Onychodactylus. 
In this small Japanese Salamander (see my second memoir on the Urodelous Skull, 
recently published in the ‘Linnean Transactions’) the skull is arrested at a very 
similar stage to that of the gigantic kind, and of the Menopome, and corresponds 
very closely with that of the Newt during metamorphosis (Pl. XL. figs. 5-7). 
What I have just remarked as to the formation of the “ thyrohyals” will be clearly 
elucidated by reference to my figures of these parts in Onychodactylus (op. cit. 
pl. xix. fig. 3, 0.7’). 
SIREN LACERTINA. 
a. Young specimen, 133 inches long. 
6. Adult, 18 inches long. 
The skull of Siren resembles that of the other Eel-like Urodela, viz. Proteus, 
Amphiuma, and Menobranchus, but differs from each and all of these in several curious 
particulars. It is flat; and its general outline forms three fifths of a long ellipse. 
A. The Investing Bones of the Skull. 
The parietals (Pl. XX XVIII. fig. 5, and Pl. XXXIX. fig. 1, p) are relatively very 
large; they extend from the line of junction of the occipital and auditory territories 
behind, to the line of junction of the ethmoid and septal regions in front. 
These bones are gently emarginate at all the parts of their external border; and 
each bone is produced externally into three projections. The first of these is gently 
hooked outwards, and lies over the sharp epiotic horn (ep); the next is a short triangle, 
it lies on the prootic region (pr.o), between the orbit and the shallow temporal valley ; 
and the third binds on the limited ethmoidal wing of the trabecula (#7). 
The hinder margin of each bone is also gently emarginate; the two bones meet, and 
have their edge scooped for muscular attachment, close over the front of the narrow 
occipital ring. 
The sagittal suture is open, and is a little longer than the frontal; the parietals run 
on and form the outer half of the roof, parallel with, and as far as the frontal suture 
is exposed; they do not, however, cover the endocranium (sp.é) perfectly, but leave a 
considerable part naked, just as they do over the auditory capsules behind. 
The frontals (jf) are only half the size of the parietals; they wedge in between those 
Vou. X1.—Ppart vi. No. 3.—January, 1882. 26 
