BY J. T. WILSON. 143 



T fully admit the force of such an objection and recognise that 

 if the view advocated in this paper is to be regarded as valid and 

 satisfactory it must be supported by an explanation of the common 

 relationship just referred to and one in which the admitted facts 

 of that relationship are not left out of account. 



In attempting to afford such an explanation, reference may in 

 the (irst place be made to the condition which is common in 

 reptilian forms. The late Prof. W. K. Parker has shown in his 

 Monograph on the structure and development of the skull in 

 Tro]ndonotus natrix (10) that the vomer occupies a position and 

 relation alike to the cartilaginous nasal septum, to Jacobson's 

 organ, and to the opening of the duct of the latter which corres- 

 ponds to the site of the naso-palatine foramen, exactly similar to 

 the position and relation which we find the dumb-bell bone to 

 occupy in relation to these structures in Ornithorhynchus. And 

 identical relations of the vomer may be recognised in others of 

 those reptilian forms in which the organ of Jacobson reaches so 

 high a degree of development. It is then the vomerine element 

 and not the premaxilla which in lower vertebrates possesses those 

 relations which among the majority of adult mammals seem to 

 be possessed by an inner or mesial osseous element of the pre- 

 maxilla. 



Professor Howes has drawn attention to the significance in 

 Caiman niger of the very exceptional arrangement due to the inter- 

 calation of the bullous anterior free extx'emities of the vomers in 

 the premaxillo-maxillary region of the palate. He has shown 

 reason for the belief that this bullous palatine lobe of the vomer 

 is to be regarded as the representative of the osseous investment 

 of Jacobson's organ generally present in other reptiles and in 

 niauiraals. In other crocodilian forms, in which the palatine lobe 

 of the vomer is absent, he found the anterior truncated extremity 

 of the vomer buried in a powerful ' vomerine ' ligament which 

 runs forward to the premaxillary region, where its fibres are 

 attached to the periosteum of the premaxillary region and to the 

 palatine process of the premaxilla when such is present. In a 

 young Alligator mississippiensis he found the fibres of this liga- 



