722 SNOUT OF FCETUS OF MONOTREMES, 



cartilaginous septum intercalates itself into the nasal floor to 

 constitute the '• prenasal plate." Fig. 17 shows a coronal section 

 immediately behind the posterior edge of the fenestra, whilst the 

 section shown in fig. 16 passes through the fenestra. 



A corresponding view of Model ii., representing the same region 

 of the snout in the larger "foetal" Ornithorhynchus is shown in 

 fig. 5. Here the fenestra is less regularly oval than in the earlier 

 stage. It is becoming vertically more compressed in front. 



The same region in the "foetal" Echidna is illustrated by fig. 9 

 from Model iii. In its stage of development this specimen 

 corresponds fairly closely to the younger of the Ornithorhynchus 

 specimens. Both as regards the condition of the septum and of 

 the prerostral notch, Echidna at this stage deviates less from the 

 ordinary mammalian condition than does Ornithorhynchus, though 

 there is no essential difference between the two Monotremes. 



It was long ago noted by the late Professor W. K. Parker (8) 

 that an internasal fenestra like that now described in the Mono- 

 tremes is a common feature in low Eutheria. He suggests that 

 it is the posterior margin of such a fenestra which represents the 

 true anterior limit of the septum nasi. We therefore see that it is 

 the specialisation of a not uncommon mammalian character which 

 accounts for much that seems highly peculiar in the Monotreme 

 snout. 



I need hardly say that the septal cartilaginous fenestra above 

 described has nothing whatever to do with the internasal defect 

 in the septum of the living Monotreme. The latter is situated 

 altogether posterior to the plane region represented by the present 

 Models. 



In fig. 2 we have a view of Model i., from the front (the " os 

 carunculse " has been removed from the model in order to allow 

 a view of the anterior parts of the snout-cartilage). On either 

 side the lower part of the anterior margin of the septum is seen 

 to expand and to become continuous ventrally with the great 

 cartilaginous sheet prolonged from the nasal floor into the 

 marginal cartilage of the upper lip. As far back as the plane of the 

 anterior end of the cartilaginous capsule of the organ of Jacobson 



