BY PROFESSOR J. T. WILSOX. 729 



serves to complete the proof originally brought forward by the 

 writer to establish the independent prevomerine character of a 

 much discussed skeletal element. The reconstructions now fisfured 

 afford complete demonstration of the fact that, quite independent 

 of the prevomer we find a genuine palatine process of the pre- 

 maxilla. This entirely harmonises with the results of the exten- 

 sive investigations of W. K. Parker upon the skeletal constitution 

 of this region. The fact that Schwink (15) failed to find evidence 

 of a compound origin of the mammalian palatine process, except 

 in one case, may very probably be explained b}^ the fact that in 

 most mammals in which the premaxilla is well developed, the 

 union of the prevomer and the palatine process proper, takes place 

 very rapidly, so that the stage of distinct ossific centres is a very 

 transient one, if, indeed, it can be said to occur at all as a rule. 

 Here in Ornithorhynchus it is only the arrest in development of 

 the palatine process proper and its progressive absorption in an 

 anterior direction, combined with a rapid lengthening of the 

 snout region and a probable demand for the preservation of the 

 prevomerine element as a skeletal support in the region of 

 Stenson's ducts, which determine the preservation of the isolated 

 posterior element of the so-called palatine process, whilst prevent- 

 ing fusion with the disappearing anterior element or genuine 

 palatine process. 



III. — The OS caruncul^, a specialised prenasal portion of 



THE MONOTREME PREMAXILLA. 



The caruncle in the young Ornithorhynchus has been recognised 

 since Owen first drew attention to it in 1865 (I6). The original 

 idea that it might function in some way similiar to the "egg- 

 breaker " of some Sauropsida has finally been set aside by the 

 more recent discovery of a true "eggs-tooth" developed upon the 

 upper lip, in younger stages of development than had previously 

 been examined (c/. Seydel, loc. cit. (9), and also Taf. x. of Semon's 

 illustrations of young of Echidna (17))- 



In a previous paper (2) I have figured the caruncle in the 

 younger of the two mammary foetuses of Ornithorhynchus now 

 48 



