1874.] PROF. T. H. HUXLEY ON MENOBRANCHUS. 193 



The posterior ceratobranchial (Ch 1 ) 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.b*). The slender, slightly 

 curved, third epibranchial(^p.6 3 )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 pro- 

 minent ; 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 attach- 

 ment 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 exten- 

 sively 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 epi- 

 branchial, 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 XXXI. fig. 3). There is a very large pituitary space, 

 bounded by the trabeculse (Tr) at the sides. In front, the latter 

 * Siren and Amphiuma hare epiotic processes of a different form. 



