236 DR R. BROOM ON THE 



directly into the mouth, as the little pocket into which it opens in common with the 

 naso-palatine canal is frequently so shallow that the ducts practically open independently 

 into the mouth. 



Behind the plane represented by fig. 2 the organ becomes much less expanded 

 laterally, while the outer part of the cartilage of the organ becomes united with the 

 lower part of the inner portion, forming a complete capsule to the organ ; and the organ 

 becomes almost cylindrical instead of flat as in the anterior part. Fig. 3 illustrates a 

 section across the posterior part of the organ. The turbinal plate, which in the anterior 

 and middle region is moderately flat, here passes upwards and inwards, and then down- 

 wards, being to a considerable extent folded on itself. Below the prevomer, which here 

 attains its maximum development, is seen a large thin plate of cartilage (o.n.f.c.) 

 stretching across from one side to the other, but distinct at the sides from the alinasals. 

 This cartilage is a backward continuation of the part of the nasal floor cartilage on the 

 outer side of the naso-palatine canal, which, on passing backwards, becomes distinct from 

 the alinasal, and sweeping inwards below the prevomer unites with the cartilage of 

 the opposite side, and forms the large plate which supports the back part of the 

 dumb-bell. 



It is exceedingly difficult, as already remarked, to pick out which characters of the 

 organ are inherited from reptilian ancestors and which are specialisations peculiar to the 

 genus or order. Unfortunately the order of reptiles, which probably contained the 

 ancestors of the Monotremes — the Theromora — is long since extinct, and the only living 

 reptilian order in which the organ is known to be well developed — the Squamata — is 

 only but distantly related to the Monotremes. Among lizards the organ of Jacobson is 

 usually very well developed, but there are great differences in the structure and relations 

 of the organ in different groups. In the Varanidae and in snakes a most complex and 

 highly specialised arrangement is found ; in the Scincoidse a different mode of 

 specialisation is met with ; while in the Agamidse and Geckonidse, on the other hand, 

 though the organ is well developed, it is comparatively simple both in structure and its 

 relations, and it is in these latter simple Lacertilian types that we recognise some affini- 

 ties to the organ of the Monotremes. In the Gecko we find the opening of the organ 

 situated inferiorly, almost exactly as in Ornithorhynchus, while we have an even more 

 remarkable resemblance in the presence of a large cartilaginous turbinal process in - 

 vaginating the organ from the outer side. It would thus seem probable that the mode 

 of opening of the organ and the well-marked turbinal in Ornithorhynchus are reptilian 

 heirlooms. The anterior extension of the organ would seem at first sight to be also a 

 reptilian character, seeing that the organ extends slightly in front of the duct in lizards, 

 but it is possible that the specialisation of the beak may be sufficient to account for this 

 peculiarity, and until the very early development is known it had better be regarded as 

 a doubtful reptilian character. Another reptilian character is to be noticed in the 

 prevomer or dumb-bell shaped bone being quite distinct from the premaxillary, and not 

 united with it as a palatine process, as is usually the case in mammals. 



