1891.] ANATOMY OF ORNITHORHYNCHUS, 581 



by Turner (4), that in this animal the premaxillarj bone consists 

 of two entirely distinct and separate portions, an outer and an inner. 

 Of these two the inner unites with its fellow of the opposite side to 

 form a small dumb-bell-shaped bone, I'os paradoxe of Albrecht. 

 As the Ornithorhynchus is the only mammal in which this arrange- 

 ment of the premaxillary bone exists, it appeared to me that its 

 position and relations were worthy of a more detailed examination 

 than they have yet received. J. F. Meckel and Owen have described 

 its general form when looked at from below, while Albrecht and 

 Turner have shown its relations to the naso-palatine foramen. I 

 propose now to demonstrate from my serial sections its relations to 

 the cartilages of the nose and to Jacobson's organ. 



When looked at from below, this bone is seen to consist of two 

 rounded nodules placed one in front of the other and united by a 

 narrow portion. Of the two nodules, the anterior is the smaller. 

 The naso-palatine foramen lies just external to the constricted portion. 

 Its total length in the adult is about 7 mm., and the breadth of the 

 posterior swelling 5 mm. 



The anterior nodule does not reach quite so far forward as the 

 nostrils, but extends a little beyond the anterior end of Jacobson's 

 organ. On transverse section (see fig. 1, PL XLIII.) it is seen to 

 form a thin layer of hone, flattened from above downwards. It is 

 separated from the cavity of the mouth by the mucous membrane 

 of the hard palate and some loose connective tissue, while its upper 

 surface lies in close relation to the cartilages of the nose which 

 contain Jacobson's organ. As it passes backwards it gets thicker 

 and develops a median ridge on its upper surface, which passes 

 upwards between the two plates of cartilage containing Jacobson's 

 organ. Opposite the naso-palatine foramen its vertical extent is about 

 twice that of its transverse, and its external surfaces are concave 

 and lodge the inner part of the cartilage of Jacobson's organ. 



A section through about the middle of the posterior nodule of 

 the bone shows that it possesses the same general form as at the 

 naso-palatine foramen. It appears here as composed of two crescents 

 with their convexities directed inwards, and the greater breadth of 

 the bone is due to the elongation of the horns of the crescents, 

 which reach about halfway round the organ of Jacobson and are 

 in close contact with its cartilage. 



In the greater part of its extent the bone is covered on its oral 

 aspect by mucous membrane and a thin layer of submucous tissue, 

 but near its posterior extremity also by a layer of cartilage. This 

 is effected by an extension inwards to the middle line of the plates 

 of cartilage which at the naso-palatine foramina lie external to 

 these openings. 



The dumb-bell-shaped bone from its position in relation to the 

 cartilages of the nose is evidently ossified in the membrane investing 

 them. In some places the ossific process has involved the whole of 

 the tissue up to the cartilage, so that the bone and cartilage are in 

 direct contact ; as a rule, however, there is a thin layer of connective 

 tissue betweoi the bone and cartilage. 



