COMPARATIVE ANATOMY OF THE MAMMALIAN ORGAN OF JACOBSON. 241 



slightly in front of the point where the naso-palatine canal ends. In every other 

 respect the characters agree perfectly with the Marsupial type, and the peculiarity is 

 probably due to the lengthening of the front of the snout in connection with the well- 

 developed front incisors. It will be noted later that the opening of Jacobson's organ 

 into the nasal floor in front of the naso-palatine canal is one of the most noticeable 

 characteristics of the rodents, and it is interesting here to notice a parallel development 

 in an animal which to a considerable degree approximates to the rodent type of dentition. 



Edentata. 



With the exception of Parker's Monograph on the development of the skull in the 

 Edentata, practically nothing has been published on the organ of Jacobson in this order, 

 and Parker's figures, though showing the presence of the organ in different forms, do 

 not enable us to form any idea of the more delicate relations. Symington, who has 

 made sections of the snout of the Peba armadillo, and of a foetal 3-toed sloth, but has 

 not yet published his results, kindly informs me that he finds the organ well developed 

 in the armadillo, and that in the sloth it is rudimentary, and opens into the nasal cavity. 

 Through the kindness of Mr F. E. Beddard, F.R.S., I recently obtained the head of an 

 adult hairy armadillo (Dasyjjus villosus), and have since made a study of the organ of 

 Jacobson and its relations in that form. In this species the organ is moderately well 

 developed, and though it is possible that in such a varied group as the Edentata there 

 may be some considerable variations in the relations of the organ, Dasypus villosus may 

 provisionally be taken as the type of the order ; and, judging by the structure in this 

 form, it seems probable to me that the other genera will not depart very greatly from 

 the Dasypus type. 



Dasypus. — In a short paper recently published I described the condition of the 

 nasal floor cartilage in its anterior region, and more especially the remarkable little 

 nasal floor bone which is associated with it. The cartilage differs in some respects from 

 that of any of the lower mammals, and also from the majority of the higher forms. In 

 most mammals the nasal floor cartilage arises as two lateral plates from the base of the 

 nasal septum : here, in front, they appear to rise by a splitting up of the lower third of 

 the nasal septum. In the plane passing through the anterior part of the papilla the 

 cartilages are quite below the base of the septum, and do not form any floor to nasal 

 cavity, the floor being formed by the little nasal floor bone. 



Fig. 16 represents a transverse section near the middle of the papilla. Here the 

 nasal floor cartilage has almost its normal development, for though the outer and inner 

 parts appear detached by the posterior part of the nasal floor bone, they are quite united 

 round behind the bone. The nasal floor cartilage and bone both rest on the peculiarly 

 flattened out premaxillary. The inner part of the nasal floor cartilage is very large, and 

 is seen curving upwards and outwards almost exactly as in Dasyurus ; there is here, 

 however, practically no inferior septal ridge, the large glandular ridge being apparently 

 the homologue of the upper of the two ridges in Echidna, and not of the lower, which 



