BY PROFESSOR J. T. WILSON. 723 



(about level of s.n. in fig. 3), th emarginal cartilage is directly 

 continuous with the cartilage of the nasal floor, of which, indeed, 

 it forms the lateral and anterior expansion. Behind that plane, 

 however, it is severed from the nasal floor cartilage — the maxilla 

 and premaxilla coming to intervene between the two — and is 

 prolonged backwards as a free marginal strip to a point which at 

 this stage is considerably posterior of the plane of the anterior 

 border of the maxillary palate (fig. 3). In front, the marginal 

 cartilage projects forwards on each side into the 11^3, considerably 

 in front of the plane of the free anterior margin of the septum 

 nasi — i.e. J of the primitive anterior end of the snout. 



There is thus formed a deep notch between the marginal 

 cartilages of opposite sides. This prerostral notch is the original 

 representative of the oval gap which Broom (7) found in the adult 

 arresting the rostral or prenasal cartilage in the middle line in 

 front. At this stage in development the notch lodges the slender 

 anterior continuations of the premaxillse which connect the latter 

 with the "OS carunculse" {v. infra). The fibrous bands which 

 Broom discovered passing from this region to the vestigial 

 caruncle of the adult can be nothing else but vestiges of these 

 lost prem axillary trabeculse, together with (and perhaps chiefly) 

 remains of the os carunculse itself. 



Only one of the two premaxillary trabecuhe is shown in this 

 figure occupying the prerostral notch, for the sake of clearness. 

 The ventral view of the model in fig. 3 may be consulted in this 

 connection, in which both the prerostral premaxillary bars are 

 shown, whilst, also for greater clearness, the prerostral extension 

 of the marginal cartilage on the right of the figure has been 

 removed. 



The complete cartilaginous ring surrounding the external narial 

 aperture and the upper of the two osseous splints representing 

 the body of the premaxilla are both well seen in fig. 2. Compare 

 figs. 4 (Model i.), 7 and 9 (Model ii.), and figs. 11 and 13 

 (Model iii.). It will be seen that in the Echidna the cartilage 

 of the aperture is incomplete. 



