558 MAMMALIAN PRENASAL CARTILAGE, 



In fig. 1, PL xliv., is represented a longitudinal median section 

 of the snout of Ornithorhynchus which shows the relations of the 

 cartilages. The nasal septum proper [n.s.) is seen in front to 

 divide into the upper delicate cartilaginous process (a.n.s.) and 

 the lower well developed prenasal {2:).n.). The upper process 

 passes l^etween the anterior nares, whose position is indicated by 

 the dotted lines (a.n.), and is probably the homologue of the 

 anterior j)art of the nasal septum in the higher mammals, as to it 

 are attached, in its hinder part at least, the alinasals. Though 

 in the region of the anterior nares it is considerably removed 

 from the prenasal plate, on passing forward it approaches the 

 latter, and ends in close connection with it, at the point where 

 the prenasal becomes arrested in the middle line. This point 

 (marked *) probably represents the anterior end of the beak in the 

 ancestor of the Platypus, as not only do the two cartilages here 

 end together, but this is practically where the two premaxillaries 

 would meet if they came together. Connected with i*, moreover, 

 there is a further feature of importance. On the upper side of 

 the beak in the middle line is a small area of thickened epithelium 

 (c) unlike that of the rest of the beak. This is probably the 

 remains of the caruncle. If it be so, and it is quite percei3tible 

 externally in the fresh specimen even to the naked eye, it corrects 

 the statement of Owen's"^ that no trace of the caruncle can be 

 found in the adult. Whatever be its signification, it is evidently 

 closely related to the supposed apical point of the primitive beak, 

 as a series of fibrous bands pass from it to the latter. 



In the anterior part of the section is seen the anterior union of 

 the incurving lateral portions (pn^), and between this and the 

 supposed apical point the cartilage is seen to be absent. 



The same section illustrates a few other points of interest. 

 The prevomer or dumbbell-shaped bone (P.vo.) is seen cut longi- 

 tudinally. Immediately in front of it Jacobson's cartilage (J.c.) 

 may be noticed at the place where towards the anterior part of 

 the organ the cartilages meet in the middle line. Behind the 



* R. Owen. " Monotremata." Todd's Cyclop. Anat. and Phys. 



