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Show 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. O n 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 pterygoid 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 Menobranchus, and far apart from the antorbital process (A.O) (the intermediate 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 Menobranchus. 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 Amphibien," 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 suspensorium ; 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 basisphenoid 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 Streptostylica and Crocodiles." t A corresponding process exists in Proteus, Siredon, Menopoma, and Am- 'DflilbTTlCl. t See m y Croonian Lecture (Proc. Roy. Soc. 1858), and, for full details, Mr. Parker's Memoir in the ' Philosophical Transactions' for 1871, already referred to. |