144 ON THE "dumb-bell-shaped" BONE IN ORNITHORHYNCHUS, 



ment partly in continuity with the walls of two fibro-cartilagiuous 

 sacs lying within the embrace of the prepalatine foramina, and he 

 considers it justifiable to assume that these sacs, together with the 

 vomerine ligament, form the vestigial remains of the palatine lobes 

 of the vomers in Caiman niger with their associated structures. 

 I have referred to these observations in detail because the descent 

 of the anterior lobes of the vomers into the prepalatine region in 

 Caiman niger presents a very fair analogy to the descent of the 

 dumb-bell ossification into the prepalatine region in Ornithorhyn- 

 chus. In the latter case, however, the ' palatine lobe ' is not 

 absolutely continuous with the main body of the vomer, the con- 

 tinuity being interrupted by the intervention of the bilateral 

 vomerine ligament which has been described above.* 



Turning next to the arrangements in the class Mammalia, we 

 may enquire whether developmental conditions amongst other 

 mammalian orders bear out the objection, founded upon the 

 adult condition of the mammalian skull, to our considering the 

 dumb-bell bone as vomerine rather than premaxillary. And this 

 question may, I think, be answered in the negative. A study of 

 Parker's elaborate monographs, especially those dealing with the 

 development of the skull in Edentata and Insectivora (12), has 

 tended strongly to confirm the idea I had previously formed of 

 the vomerine nature of the bone in question. 



Attention may be specially directed to the following amongst 

 Parker's figures — Tafusia hyhrida, PL ir. fig. 6 v.', along with the 

 transverse sections in PI. iii. figs. 7 and 8 v'. A later stage of 

 the same, showing the palatine anterior vomers, is figured on his 

 PI. V. fig. 5 lettered, by mistake, o'. 



In the ripe embryonic condition represented on his PL vi., fig. 1 

 shows the palatine anterior vomers considerably restricted in 

 extent and forming that part of the palate which intervenes 

 between the naso-palatine foramen and the " mesial palatal 

 suture." At this stage, however, they appear to be fused with 

 the palatine plate of the premaxillse. 



* This bilateral vomerine ligament is actually figured by Meckel (2, PI. vii. 

 fig. 11), though his description of the bone and its relations is meagre. 



