58 



ON THE SKELETON OF THE SNOUT AND OS CAR 



UNCULJE OF THE MAMMARY FCETUS OF 



MONOTREMES. 



By Professor J. T. Wilson, M.B., Ch.M. 



For the research three specimens were utilised. One was the 

 ftetal Omithorhynchus, whose external characters were described 

 by the writer in a previous paper before the Society. Another 

 was a more advanced specimen of Omithorhynchus, whilst the 

 third was an Echidna of about the same stage as the earlier of 

 Professor W. N. Parker's specimens. All the stages were more 

 advanced than those of Echidna lately investigated by Seydel. 

 Wax-plate reconstructions of the anterior snout region were 

 exhibited together with serial photographs of the younger Omitho- 

 rhynchus. 



The following features are revealed and illustrated by the 

 models : — (1) The complete continuity of the nasal floor cartilage 

 and the extensive marginal cartilage of the upper lip, which in 

 the adult are separated by the premaxilhe. (2) As a result of 

 this continuity the premaxilla? arise each as two entirely distinct 

 4)ony splints on the dorsal and ventral surfaces of the cartila- 

 ginous plate aforesaid. (3) The great forward expansion of the 

 so-called rostral cartilage of the Omithorhynchus is seen to be 

 due to the forward growth of two bilateral alar expansions of the 

 same cartilage, which tend to meet in front, after enclosing a 

 deep notch corresponding to the hiatus described by Broom in the 

 rostral cartilage of the adult. (4) The ventral lamella? of the 

 premaxillse are provided with true palatine processes directed 

 backwards paramesially. In the older of the two stages of 

 Omithorhynchus there exists, quite independently of the palatine 

 process, and separated from it by a considerable interval, a 

 separate ossification for the dumb-bell-shaped bone, which is thus 



