OKGAK VF JACUDSON JK Si'HENODON. il7 



m structure or relationships, are comparatively slight. The 

 organ is asually of large size, and has a well-developed turbiual 

 process passing into it from its anterior and under side, and it 

 is roofed by the lai'ge septo-maxillary bone. It usually opens 

 more or less directly into the mouth, and it is only indirectly 

 connected with the nasal cavity. In the Greckouidae, whicli are 

 probably the most primitive of living lizards, the organ opens 

 with the lachrymal duct into a cavity which is connected with 

 the nasal chamber, and iu this group the septo-maxillary, though 

 it forms the roof of the organ and articulates with the nasal 

 septum, is considerably smaller than in the typical lizards. The 

 Gecko thus shows a nearer affinity with Sphenodon than do the 

 other lizards, and helps us to understand how the lizard type of 

 organ may have developed i'rom something like the Sphenodon 

 type. 



In the way in wliich the organ opens into the anterior j)art of 

 the lower portion of the nasal cavity^, the resemblance is much 

 closer to the mammalian condition than to the lacertilian. In 

 the mammal the resemblance is slightly obscured by the forma- 

 tion of the secondary palate by which the anterior part of the 

 nasal cavity becomes the naso-palatine canal. In the mamma, 

 the organ of Jacobson normally opens into the naso-palatine 

 canal. This condition is found in the Mouotremes, all Mar- 

 supials (except yEpyprymnus and Notoryctes), Ungulates, Car- 

 nivores, Insectivores, Bats, and Lemurs. It does not occur, 

 however, ni Rodents, Basypus, or Man. The roofing of the 

 organ by cartilage, though exceptional in Mammals, is by no 

 means uncommon. It is met with in the Monotremes, a few 

 Marsupials, and a number of Eutherians. In Mammals the 

 organ only very exceptionally extends in front of the opening. 

 In Oriiithorhynchus, however, about as much of the organ lies 

 in front of the duct as behind it, and the way in which the 

 anterior part of the ofgan excavates the lateral cartilage is not 

 unlike the condition seen in Sphenodon ; and in Ornithorhynchas 

 the cartilage is supported by the prevomer or " dum.bbell bone," 

 just as in Sphenodon it is supported by the bone usually, but I 

 believe erroneously, called the " vomer." In no higher mamniaL 

 does the organ extend much in front of the duct, but it does to 

 a sli'^ht extent in Feranieles and Macroscelides, A striking 

 difference between the organ in Ornithorhynchus and Sphenodon 

 is that in the former there is a well-developed turbiual passing 



