1874.] PROF. T. H. HUXLEY ON MENOBRANCHUS. 203 



termed the " pylangium' '* : . The bulb-like termination of the truncus 

 arteriosus, on the other hand, is divided into four canals by two septa 

 disposed at right angles to one another, and is clearly formed by the 

 coalescence of the aortic arches. It may therefore be termed the 

 " synangium." 



Throughout the Amphibia these two constituents of the truncus 

 arteriosus are readily distinguishable, though they vary very much in 

 form and proportions, the pylangium being longest in proportion to 

 the synangium in the Anura, shortest in the Peromela. Moreover, 

 in the higher Urodela and the Anura, the pylangium becomes com- 

 plicated by the development of a longitudinal septum, which extends 

 from the anterior to the posterior set of valves, and imperfectly 

 divides the cavity into two chambers. 



DESCRIPTION OF THE PLATES. 



Plate XXIX. 



Fig. 1. Side view of the skull of Menobranchits lateralis. 

 2. Ventral view of the same. 



Plate XXX. 



Fig. 1. Dorsal view of the same, all bony matter being removed from the left 

 half of the skull. 



2. Ventral view of the hyoidean and branchial apparatus. 



All the figures of the skull are magnified three times. In all, carti- 

 lage is coloured blue, and cartilage-bone light brown. Membrane-bone 

 is left uncoloured. 



The signification of the letters is the same throughout. 



Cartilage-bones. — E. 0, exoccipital; Ep.O, epiotic and opisthotic; 

 Pr. 0, prootic ; Qu, quadrate ; St, stapes ; Bb 2 , second basibranchial. 



Membrane-bones. — Pa, parietal, with its three processes Pa 1 , Pa 2 , Pa 3 ; 

 Sq, squamosal ; Fr, frontal, with its antero-lateral prolongation, Fr. 1 ; 

 Pmx, prwmaxilla ; Vo, vomer ; Pa, Pt, coalesced palatine and ptery- 

 goid ; P.S, parasphenoid ; B, dentary ; Sp, splenial. 



Chondrocranium. — 7V, trabecula ; A.o, antorbital process; q, quad- 

 rate process ; p, pterygoid process ; a, ascending process ; o, otic pro- 

 cess ; m, pedicle of the suspensorium ; Mck, Meckel's cartilage ; Hy, 

 hyoid arch ; Hh, hypobyal ; Ch, ceratohyal ; BW , first basibranchial ; 

 Cb x , first ceratobranchial ; Epb 1 , first epibranchial ; Cb 2 , second cerato- 

 branchial ; Epb 2 , Epb 3 , second and third epibranchials. 



Ligaments. — m.h.l, mandibulo-hyoid ; h.s.l, hyo-suspensorial ; s.s.l, 

 suspensorio-stapedial. 



Nerves and foramina for nerves ; I, olfactory foramen ; V, orbito- 

 nasal branch of the trigeminal ; Y 2 < 3 , second and third division of the 

 trigeminal ; YII.p, posterior division of the portio dura. 



Plate XXXI. 



Fig. 1. Chondrocranium of a larva of Siredon. Fig. 2, of Triton, ventral 

 aspect ; fig. 2 a, hyoidean and branchial apparatus of the same Triton- 

 larva. 



In these larvae the walls of the auditory capsules were not yet 

 chondrified. Letters as before, except N, nasal sac ; Ch, notochord. 



3. Chondrocranium of a tadpole, dorsal aspect. V, trigeminal ganglion ; 



VII, ganglion of the seventh nerve. 



4. The suspensorium of Menobranchits ; and fig. 5, that of Sana escidenta, 



ttvXii, a gate, and ayyelov a vessel. 



14* 



