416 n]{, 1!. BKooisr on thr 



organ, but never shows on the outer surface of the skull as in 

 Sphenodon. In snakes the septo-mnxillarv is more largely 

 developed than in lizards, and to a considerable extent supports 

 the nasal septum beyond the limits of the organ of Jacobson. 



A septo-maxillary bone has not hitherto been described as 

 such in any mammal, but it appears to occur in the immediate 

 ancestors of the Mammals, the Cynodouts, and is known to bo 

 present in the more primitive Therocephalians and Pelyco- 

 saurians. No trace of the bone has been found, however, in 

 any Anomodont. In 1896 I discovered a nasal-floor bone in 

 the Armadillo {Basi/pus villosns), but did not at tlie time 

 recognize its true significance. Fig. 9 represents a section 

 through the bone; and if this be compared with figs. 2 & 3, it 

 will be seen that there is considerable reason for regarding the 

 " casal-floor bone " of the Armadillo as homologous with the 

 septo-maxillary of Sphenodon. The fact that the septo-maxillary 

 is seen from the Splienodon condition to be primarily a nasal- 

 floor bone, and that the bone occurs in the mammalian ancestors, 

 renders it exceedingly probable that the bone in Dasyjnis is a 

 true septo-maxillary- 



The organ of Jacobson, as illustrated in figs. 3, 4, 5, & G, is 

 seen to be relatively much less developed tlian in lizards and 

 snakes, and to differ very much from the type found in the 

 Squamata. It is a flattened lens-shaped organ which lies near 

 the base of the nasal septum, between the prevomer and the 

 nasal cavity. The anterior part of the organ, as seen in figs. 3 

 & 4, is supported below by the paraseptal cartilage, and above 

 by a special roofing-cartilage. The plane of the organ looks 

 upwards and slightly outwards. At a point near the middle of 

 the outer side, the organ opens into the anterior end of the 

 lower part of the nasal cavity, as seen in figs. 4 & 5. The 

 lachrymal duct opens into the same part of the nasal cavity from 

 the outer side. On passing farther backwards the organ is seen 

 to be rather narrower than in front (see fig, 6). It ends quite 

 abruptly and ]iot, as is the rule in Mammals, by becoming a 

 gland-duct. 



The differences between the organ in Sphenodon and that in 

 the typical representatives of the Squamata are much greater 

 than might have been expected, considering the many lizard- 

 like characters 0^ Sphenodon. In tlie Lacertilia and Ophidia the 

 organs are formed on a common type, and the difference's, either 



