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Show 1874.] PROF. T. H. HUXLEY ON MENOBRANCHUS. 193 The posterior ceratobranchial (Cft) is a mere nodule of cartilage, which is connected, externally, with the anterior ceratobranchial and first epibranchial, and on its dorsal side articulates with the enlarged ventral end of the second epibranchial (Ep.b2). The slender, slightly curved, third epibranchial(Z?p.63) articulates with the enlarged ventral end of the second. There is no trace of a fourth epibranchial. On comparing the cranium of Menobranchus with that of other Amphibia, one is at once struck (as Van der Hoeven has already remarked) by its many resemblances to that of Proteus. In Proteus, the skull is similarly elongated and narrow, especially • in the nasal and maxillary regions. The epiotic processes are prominent ; and the suspensorium is inclined downwards and forwards at a like angle. The nasal, maxillary, and jugal bones are absent in Proteus, as in Menobranchus; the vomers and the palato-pterygoids have a similar disposition. In the general form and mode of attachment to the skull, in the rudimentary condition of the posterior ceratobranchial, in the presence of only three epibranchials, the hyoid and branchial apparatuses of Proteus closely accord with those of Menobranchus, though those of Proteus are much more extensively ossified. In both genera the epiotic and opisthotic regions ossify and give rise to a distinct bone, the summit of which forms the epiotic process. Moreover, the chondrocranium of Proteus is, in all essential respects, similar to that of Menobranchus, though the tra-beculse are partially ossified where they lie between the nasal sacs. In possessing prominent epiotic ossifications, which project as strong conical processes from the occipital region of the skull, Menobranchus and Proteus differ from all other existing Amphibia, and agree with the extinct Labyrinthodonts *. In the absence of the fourth epibranchial, Proteus and Menobranchus differ from Siren, Siredon, Menopoma, and Amphiuma. In the rudimentary condition of the second ceratobranchial they approach Amphiuma, in which this element is absent. In the structure of the chondrocranium, Menobranchus and Proteus differ from the Frog and from Siredon (the only Amphibia in which the chondrocranium has as yet been thoroughly examined) in the persistence, throughout life, of a far more embryonic type of structure. In fact, the skull of even the Lamprey is, in some respects, less embryonic than that of Menobranchus, the floor and roof of the occipital region having acquired a more complete chondri-fication in the Marsipobranch. It is to the embryonic condition of the vertebrate skull, especially in the class to which Menobranchus belongs, that we must have recourse for an explanation of the structure of its primordial cranium. If the cartilaginous skull of a tadpole, before it has lost its external gills, be compared with the persistent chondrocranium of Menobranchus, the general correspondence of the two becomes obvious (Plate X X X I . fig. 3). There is a very large pituitary space, bounded by the trabeculae (Tr) at the sides. In front, the latter * Siren and Amphiuma have epiotic processes of a different form. |