732 SNOUT OF FCETUS OF MONOTREMES, 



aspect of the cartilaginous nasal floor (n.f.) and {al.n.), which are continuous 

 in front and laterally with the marginal cartilage (??i.c.). 



s.n'., septum nasi where it descends and is intercalated into and continuous 

 with the cartilage of the nasal floor; n.p.c. , naso-palatine foramen (Stenson's 

 duct); J.C., cartilaginous capsule of Jacobson's organ, and J.c\, its posterior 

 extremity; s.n"., edge of ventral border of septum nasi which is largely 

 hidden by the anterior extremity of the vomer, vo.; al.n.t., alinasal turbinal 

 ridge projecting into the cartilaginous nasal cavity; n.d., nasal duct proceed- 

 ing along outer surface of alinasal wall of nasal cavity; n.d'., nasal duct 

 passing inwards and piercing alinasal near its ventral border; mx., anterior 

 prolongation of maxilla forming a trough which lodges the posterior end of the 

 ventral lamina of the body of the premaxilla; mx'., palatine plate of maxilla; 

 s.p.c, transverse cartilaginous lamina developed in the secondary palate, its 

 anterior margin forming the true anterior limit of the skeleton of the latter. 



Fig. 4. Eight lateral aspect of Model i. 



O.C., OS carunculae ; s.n., septum nasi, anterior border bounding septal 

 fenestra (fen.) in front; cap., cartilaginous boundary of anterior narial 

 aperture; al.n., alinasal wall of nasal cavity; n.f., nasal floor; m.c, marginal 

 cartilage of snout; px"., dorsal lamina of body of premaxilla ; n., nasal bone; 

 mx., maxilla ; 7i.n., nasal nerve ; n.d.n., notch in anterior margin of alinasal 

 in which nasal duct turns inwards to open into anterior part of nasal cavity. 



Fig. 5. Sagittal section close to left side of septum of Model ii. (snout of 

 larger foetal Ornithorhynchus). 



al.n., alinasals of right and left sides (cut edge of left); cap., cartilage of 

 narial aperture; s.n., septum nasi where it forms median bar bounding septal 

 fenestra (/en.) in front; s.n'., more posterior portion of septum nasi; 

 n.f., nasal floor, the more vertical dotted line leads to the place where the 

 septum nasi descends to be intercalated into the nasal floor; J.c , cartilaginous 

 capsule of Jacobson's organ; p.vo., prevomer or dumb-bell bone; ^.^x., pala- 

 tine process of premaxilla; px. , cut surface where ventral lamina of body of 

 premaxilla is continuous with rest of bone; px'., trabecula of left premaxilla, 

 continuous through prerostral notch with degenerating os carunculae, o.c; 

 m.c, rostral ends of left and right marginal cartilages of snout. 



Fig. 6. Anterior view of Model ii. There is a considerable amount of 

 necessary foreshortening. The ragged and irregular outline of the degene- 

 rating OS carunculas {o.c) is seen continuous with the rest of the premaxilla 

 only through the right trabecula in the prerostral notch, the left having 

 already been absorbed. The nasal duct {n.d.) is seen to turn inwards in the 

 notch in front of the alinasal cartilage {al.n'.),to reach the nasal cavity; {mx'.), 

 edge of right marginal cartilage. The rest of the lettering as in previous 

 figures. 



