194 PEOF. W. K. PAEKER ON THE STEUCTUEB AND 



than the other ; it is dilated at its fore end into a subcircular disk ; and this part repre- 

 sents, but is not segmented oiF as, a basihyal {h-hy). The bony shaft leaves a large 

 crescent unossified in front, but very little behind (b.br^). 



The second {h.h^) bar is much slenderer, and at its middle divides into three flat 

 lobes, which are somewhat dilated at their free ends. The ceratohyals articulate with 

 the dilated end of the first basal piece ; this is suddenly narrow and compressed ; it then 

 broadens and is depressed. To this part the first branchial arch is articulated {c.br^) ; 

 there is no joint in this ; but the upper part, or epibranchial {e.hr^), is left unossified ; 

 this is broad and falcate. The ossified part, nearly half, is a rounded phalangoid shaft 

 of bone, the cerato-branchial {c.br^). 



The second cerato-branchial [c.br') is of the same length as the first, but slenderer ; 

 it is not so much ossified distally ; this bone carries the falcate epibranchial {e.br^), and 

 is articulated to the middle of its dilated lower end. 



No more ceratobranchials exist ; but the second carries on the hinder pier of its distal 

 end, a third epibranchial {e.hr^) also falcate, but smaller ; then in turn comes the fourth 

 {e.br% still smaller and falcate, but bent in the opposite direction. 



D. The Cranial Nerves of Siren lacertina. 



I have figured most of the cranial nerves — not the third, fourth, or sixth. Their 

 relations to the cranium are exactly as in Menobranchus. (See Huxley, P. Z. S. 

 Mar. 17th, 1874). 



I have already spoken of the olfactoiy nerves (i) which pass obliquely through the 

 sphenethmoid (PI. XXXIX. figs. 1, 2) to the partially roofed nasal sac. 



The optic nerves (ii) pass out through a foramen near the end of the sphenethmoid ; 

 this passage is twice the diameter of the small nerve, but is not a large fenestra as in 

 the Anura. 



The trigeminal nerve (v) sends its orbito-nasal branch (v^) under the ascending 

 process ; and the root of the second and third branches (v^, v^) passes over the pedicle 

 outside that process ; the facial nerve (vii) winds round between the capsule and its para- 

 chordal plaster ; near its root it sends forward the " vidian branch " (vii'), which runs 

 outside and a little below the orbito-nasal to the front of the face, and whilst its main 

 trunk (vii) passes out over the epihyal and hyosuspensorial ligament. 



The glosso-pharyngeal and vagus (ix, s) pass out of a common passage in the 

 exoccipital ; I do not see a dividing bar in this foramen, such as exists in most of 



the "Anura." 



E. Characters peculiar to the Siren's Skull. 



The skull of this type comes nearest to that of Proteus, Menobranchus, and Amphiuma 

 in general form, and to the larval skulls of the species of Spelerpes ; yet the larval skull 

 of Triton is more like it in some important structures, leaving out the mere form 

 (PI. XL. figs. 2, 3). 



