BY PROFESSOR J. T. WILSON. 731 



Even in the younger Ornithorhynchus the interior of the os 

 caruncuhe (fig. 22) is in part hollowed away by osteoclastic 

 absorption. Towards the dorsal portion of its interior there 

 appears in successive sections a patch which shows on high power 

 examination a structural character indistinguishable from that 

 of hyaline cartilage, partially calcified, it may be, but which has 

 not yet undergone neoplastic ossification. What the significance 

 of this small and apparently cartilaginous vestige may be, I am 

 unable to determine. Whilst embedded in, it appears tolerably 

 sharply distinguishable from, the rest of the osseous tissue of the 

 caruncle. 



The further details of the anatomy of the snout-skeleton of the 

 foetal Monotreme will best receive elucidation in the course of a 

 description of tlie plates. 



DESCRIPTION OF PLATES. 

 Plate xxxvii. 



Fig. 1 represents a view of the left side of the septum nasi and mesial 

 aspect of the interior of the cartilaginous capsule of Jacobson's organ {J.c), 

 as exposed by a sagittal section of through Model i. , to the left of the median 

 plane. The cartilaginous septal fenestra and the profile outline of the os 

 carunculas (ox.) are well seen. Note also the anterior extremity of the true 

 vomer (I'o.) and the cross section of the transverse cartilaginous lamina {s.p.c.) 

 of the secondary palate. 



Fig. 2 gives a view of Model i. , from front, foreshortened, with the os 

 carunculae removed in order to show more clearly the cartilaginous skeleton 

 of the front of the snout. Only one, the right, of the two premaxillary 

 trabeculae which are continued into the os carunculae, is figured, and that is 

 cut across at the level of the prerostral notch in which it is lodged. 



m.c, marginal cartilage of upper lip, continued behind into the cartilage 

 of the nasal floor, n.f.; .s.?i., anterior margin of septum nasi, forming anterior 

 boundary of cartilaginous fenestra; al.n., Cartilaginous roof of nasal cavity 

 formed by alinasal ; px'., premaxillary trabecula; px"., dorsal lamina of 

 body of premaxilla. 



Fig. 3. Ventral aspect of Model i. The os caruncula (o.c.) is seen in front 

 connected by the slender premaxillary trabeculae (the left one being cut short) 

 (px'.) with the ventral lamina of the body of the premaxilla (px.) and with 

 the palatine process of the premaxilla (p.px. ). These latter lie on the ventral 



