134 ON THE "dumb-bell-shaped" BONE IN ORNITHORHYNCHUS, 



greater portion of the palatine plate of the dumb-bell lying 

 anterior to this appears as if applied to the ventral surface of the 

 cartilage of the nasal floor, and peripherally it is so applied (c/. 

 figs, 1-3 for points referred to in this description). This palatine 

 plate consists of an anterior (a) and a posterior (b) moiety united 

 by a narrow isthmus (c). The isthmus is rather nearer to the 

 anterior than to the posterior end of the plate. The naso-palatine 

 foramen lies on each side opposite and close to the isthmus. 



The antero-posterior diameter of the anterior segment, measured 

 to a point opposite the anterior margin of the naso-palatine 

 foramen, is 35 mm. Its greatest breadth is 4 mm. The antero- 

 posterior diameter of the posterior segment is 4*5 mm. and its 

 greatest width is 7 mm. Its hinder border forms a nearly semi- 

 circular line. The width of the isthmus is 1'5 mm. The palatine 

 plate is comparatively thin, and t;he term " dumb-bell-shaped " is 

 only applicable to the general contour of the periphery as seen 

 from below, and indeed it only imperfectly describes that contour. 



On dissection to expose the dorsal aspect of the thin palatine 

 plate of the bone, there is seen arising from its dorsal median 

 line a vertical plate (/) which dorsally sends out diverging alar 

 laminae (g) on either side. These form the dorsal horns of the 

 "crescents" which Symington has described in transverse sections. 

 The spread of the dorsal alar laminae at its maximum is 3 mm., 

 and they do not form a horizontal plate, but from their upward 

 divergence they form dorsally a deep median groove or gutter 

 which receives the ventral edge of the cartilaginous nasal septum. 

 The maximum height of this vertical portion of the bone is 2 mm., 

 the maximum height of the bone as a whole being 3 mm. The 

 vertical plate does not extend to the anterior extremity of the 

 horizontal or palatine portion of the bone, but begins about 

 15 mm. behind the anterior end of the latter in a gradually 

 ascending cre.st with an oblique margin. Just opposite the hinder 

 end of the preaxial segment of the bone the height of the crest is 

 about 1"5 mm. Here it rather suddenly develops its dorsal alar 

 laminse, and from this point backwards it slightly increases in 

 height, though on the whole its height is fairly uniform and not 



