730 SNOUT OF FCETUS OF MONOTREMES, 



under consideration, whilst Semon has figured earlier conditions 

 of the caruncle co-existing with the " egg-tooth " in the young of 

 Echidna (17). 



So far as I am aware, no description of the skeletal basis of 

 the caruncle is extant apart from the abstract I presented at a 

 former meeting of the Society (i). 



Seydel (9) has, however, figured without remark (other than the 

 lettering " os incis." and "praem.," " unpaarer Fortsatz beider 

 Zwischenkiefer " in his text-figures 10 and 11) the os caruncul^e 

 of Echidna, which is evidently at no time so well developed as it 

 is in Ornithorhynchus. 



In fig. 3 (Model i.) the premaxill^e, or rather the inferior 

 lamellse of them (px.), are traceable forwards into the prerostral 

 region where they become attenuated and turn up dorsally into 

 the prerostral notch and in front of the anterior extremity of the 

 septal cartilage. Here the two osseous trabecul^e (px.) fuse to 

 constitute a remarkable nodule of bone (o.c.) which forms a 

 skeletal foundation for the caruncle. This latter structure, as is 

 evident both from Semon's figures of young Echidna and from 

 similar stages of Platypus, as yet undescribed, which I have had 

 the opportunity of examining, attains a relatively large size and 

 must possess some definite physiological significance, perhaps for 

 the function of lactation, as yet undetermined. In the specimen 

 represented by Model i., the os caruncula3, as I have ventured to 

 name it, has probably already passed the zenith of its development. 

 In the older specimen it is undergoing active resorption and is 

 represented by only partially-connected bony nodules, and is 

 permeated by osteoclasts. In the Echidna specimen represented 

 by Model iii., the os caruncul?e is still more rudimentary, although 

 the stage is otherwise almost parallel to the younger of the two 

 Ornithorhynchus. Its original connection with the premaxilla has 

 entirely disappeared, and it itself constitutes only an insignificant 

 oval nodule of bone placed dorsally in front of the upper part of 

 the anterior margin of the septum, remote from the rest of the 

 premaxilla (figs. 9 and 10, o.c). 



