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Show 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 complicated 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 Menobranchus 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, cartilage 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.O, exoccipital; Ep.O, epiotic and opisthotic; Pr. 0, prootic ; Qu, quadrate ; St, stapes; Bb2, second basibranchial. Membrane-bones.-Pa, parietal, with its three processes Pa1, Pa2, Pa3; Sq, squamosal; Fr, frontal, with its antero-lateral prolongation, Fr. 1 ; Pmx, praemaxilla ; Vo, vomer ; Pa, Pt, coalesced palatine and pterygoid ; P.S, parasphenoid; D, dentary ; Sp, splenial. Chondrocranium.- Tr, trabecula; A.o, antorbital process; q, quadrate process; p, pterygoid process; a, ascending process; o, otic process ; in, pedicle of the suspensorium; Mck, Meckel's cartilage; Hy, hyoid arch ; Hh, hypohyal; Ch, ceratohyal; Bb1, first basibranchial; Cb1, first ceratobranchial; Epb1, first epibranchial; Cb2, second ceratobranchial ; Epb2, Epb3, second and third epibranchials. Ligaments.-m.h.l, mandibulo-hyoid; k.s.l, hyo-suspensorial; s.s.l, suspensorio-stapedial. Nerves and foramina for nerves; I, olfactory foramen; V, orbitonasal branch of the trigeminal; V2'3, second and third division of the trigeminal; VII.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 Menobranchus; and fig. 5, that of Bana esculcnta, * TTVXU- a gate, and ayyeiov a vessel. 14* |