1874.] PROF. T. H. HUXLEY ON MENOBRANCHUS. 195 



end of the suspensorium extended up into its dorsal crus, or otic 

 process, we should have a quadrate bone, exactly like that of a Che- 

 Ionian reptile. On the other hand, if the ventral crus became ossified 

 continuously with the inner process of the pterygoid, and the basi- 

 sphenoid were developed, we should have such a connexion of the 

 pterygoid with the basisphenoid as exists in many Lizards and 

 Birds *. Whence it appears to follow, that this part of the ptery- 

 goid represents the, morphologically, dorsal end of the mandibular 

 arch, and that the dorsal end of the os quadratum is a secondary 

 development of that arch, which becomes applied to the outer face 

 of the auditory capsule. 



The articular surfaces for Meckel's cartilage are corresponding 

 points in both Menobranchus and the Frog's tadpole ; but the 

 palato-pterygoid process (p), which is rudimentary in the Meno- 

 branchus, and far apart from the antorbital process (A.O) (the inter- 

 mediate space being occupied only by membrane, bone, and connective 

 tissue) is, though equally short, completely fused with the antorbital 

 process in the tadpole. 



There remain to be compared the orbital process (Or.) of the 

 suspensorium of the tadpole and the ascending process (a) of the 

 suspensorium of Menobranchus. 



It is clear that the orbital process, if it grew upwards and inwards 

 towards the dorsal side of the trabecula, might very well cover in the 

 orbito-nasal branch of the fifth nerve, as it actually does in Meno- 

 branchus. But then it would also cover in the third division of the 

 fifth and the levator muscle of the mandible, internal and anterior 

 to which it lies in Menobranchus. 



For these reasons I do not identify the "orbital process" of the 

 tadpole's suspensorium with the " ascending process " of that of 

 Menobranchus f, though in some respects they are analogous. 



In the tadpole, the tissue on each side of the notochord is so 

 largely chondrified that it has formed a complete floor to the occipital 

 and interauditory region of the skull, has roofed in the occipital 

 region, and has coalesced with the auditory capsules ; and the skull 

 has attained this condition at a much earlier stage than that to which 

 reference is here made J. 



But I know of no condition of the skull of the Frog which is 



* Stannius ('Handbuch d. Zootomie,' 2te Auflage: "Die Amphibian," p. 36) 

 remarks, in giving the general characters of the skull in the " Amphibia Dipnoa," 

 that the more or less cartilaginous pterygoid arcade in these animals is always 

 connected with the rest of the skull in three places : — 1, with the suspen- 

 sorium ; 2, with the sphenoidal region of the skull ; 3, with the lower part of 

 the anterior and outer wall of the orbital cavity. Of these " the connexion with 

 the sphenoidal region answers to the articulation of the pterygoid with the basi- 

 sphenoid in most Streptostylica [Lacertilia and Ophidia],Birds,&c; the connexion 

 with the lower part of the anterior wall of the orbit corresponds with the union 

 of the pterygoid with the maxilla and jugal by means of an os transversum in 

 Streptostylhea and Crocodiles." 



t A corresponding process exists in Proteus, Siredon, Menopoma, and Am- 

 phiuma. 



X See my Croonian Lecture (Proc. Roy. Soc. 1858), and, for full details, Mr. 

 Parker's Memoir in the ' Philosophical Transactions' for 1871, already referred to. 



