BY J. T. WILSON. 149 



passes through the right nostril and exposes the right side of tlie 

 septum nasi. The internasal aperture is seen below the level of 

 the longitudinal ridge running backwards from Jaoobson's organ. 

 The capsule of tlie latter has been shaved by the section at its 

 most bulging part. The arrow points up towards the naso-pala- 

 tine foramen. 

 Fig. 5. — Sagittal section of snout of Echidna to left of mesial plane, passing 

 through left nostril, showmg nasal septum with its longitudinal 

 ridge and internasal aperture. 



References to Jigs, 1-5. 

 a, anterior segment ; b, posterior ; c, isthmus ; d, line on posterior seg- 

 ment indicating anterior limit of plate of cartilage which clothes the inferior 

 .surface posteriorly ; e, bifurcated vomerine spur ; /, vertical part of bone ; 

 {/, dorsal ala ; h, nostril ; ^, internasal aperture ; j, Jacobson's organ ; k, 

 marginal cartilage of upper lip. 



Plate ix. 



Figs. 6-12. — Coronal sections through snout of Ornithorhynckm. These 

 figures have been drawn on a reduced scale of one-half from 

 photomicrographs having a magnification of eleven diameters. 



Fig. 6. — T.s. in plane of posterior boundary of internasal aperture. The 

 bone clothing the cartilaginous septum ventrally is the vomer 

 (main vomer), and it is seen just meeting the dorsal crest of the 

 maxillary palate. 



Fig. 7. — T.s. through about the middle of the internasal aperture. Ventrad 

 of the septum are the forks of the vomerine spur of the dumb- 

 bell-shaped bone separated by a little fatty tissue. 



Fig. 8. — T.s. through plane of anterior boundary of internasal aperture. 

 The vomerine spur of the dumb-bell is becoming more ventrally 

 placed and is connected with the cartilage just in front of apex 

 of the median process of the osseous maxillary palate by means 

 of some fibrous tissue. The posterior extremity of Jacobson's 

 organ is cut through on either side. 



Fig. 9. —T.s. in plane of hinder end of posterior segment where that is still 

 covered below by the cartilaginous lamina of the nasal floor. 



Fig. 10. — T.s. through snout, cutting postaxial segment of dumb-bell-shaped 

 bone in front of line d in fig. 1. 



Fig. 11. — T.s. through snout in plane of naso-palatine foramina and isthmus 

 of dumb-bell bone. 



Fig. 12. — T.s. snout in plane passing through hinder portion of preaxial 

 segment and commencement of vertical crest of bone. Here the 

 organ of Jacobsqn is about its widest. 



