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Show 152 PROF. G. B. H O W E S O N JACOBSON'S [Feb. 17, sippiensis, in which I found the arese of the pre-palatine foramina to be occupied by the two small sacs before mentioned (jc. ?, fig. 4), the fibres of these vomerine ligaments could be traced to a connection with both the membranous expansions closing the former and the fibro-cartilaginous walls of the latter. The established doctrines of morphology and the rules of precedent alike allow us to regard these ligaments as the equivalents of the " missing parts," and to look upon them and the sacs of the prepalatine foramina as the vestigial remains of the palatine lobes of the vomers of Caiman niger with their associated structures. I have unfortunately been unable to obtain the head of a Caiman niger in spirit; but inasmuch as in a small C. sclerops of 125 centim.x I have found pre-palatine foramina to be present in a form identical with that of Alligator ?nississippiensis, the conclusion formulated above receives additional support. I find the pre-palatine foramina to be present in most, but not all, of the skulls of Alligator mississippiensis which I have examined smaller than the one afore dealt with in detail (total length 11.2 centim.) ; while in the skulls of two adults preserved in the Natural History Museum, which Mr. Boulenger has kindly afforded m e an opportunity of examining, I find (on the left side of the one and the right side of the other) an insignificant perforation which may perhaps be a last vestige of the prepalatine foramen (?), but this is doubtful. It is clear, from all, that the structures possessed by m y specimen here figured (figs. 5 and 6) when present must disappear with advancing age ; and it has yet to be ascertained if their like is not to be found in other allied genera and species. III. The only structure with which it is possible to homologize a cartilaginous sac in intimate relationship with the prepalatine foramen of a Reptile is the Organ of Jacobson. This has of late years received an exceptional amount of attention. Leydig has described 2 its general features and relationships in certain Lacertilia, in that masterly manner so peculiarly his own. Its more detailed characters and origin have been incidentally dealt with by Born, in the course of a series of lengthy monographs 3 devoted to a comparative study of the nasal chamber and its ducts ; while Solger, Wright, and others 4 have furnished details about it in certain reptilian forms. Concerning the Crocodilia, its absence is everywhere proclaimed ; and Beard, who has most recently investigated the facts of its early development, specially states 6 that he " searched for it in Chelonians and in embryo Crocodiles, but in vain." All recent inquiry has rendered it more and more clear that the 1 Tail broken ; measurement taken to posterior extremity of cloaca! orifice. 2 Die in Deutschld. lebend. Arten d. Saurier. Tubingen, 1872, pp. 96-99. 3 Morph. Jahrb. Bd. ii. bis, viii. (1877-1883). 4 For a full bibliography with detailed references, see Beard in Zoolog. Jahrb. Bd. iii. pp. 778-780 (1889). 5 Loc. cit. p. 772. |