BY R. BROOM. 603 



Petauroides volans; (4) adult Petaurus breviceps; (5) very early- 

 mammary foetus, Trichosurus vulpectda; (6) early mammary foetus, 

 Trichosurus ; (7) large mammary fcetus, Trichosurus ; and (8) 

 adult Tx'ichosurus. 



In all these genera the same type is followed, and the close 

 agreement between the different genera is remarkable. 



PseudochiruA peregrinus, Bodd., (mammary foetus, head length 

 8-5 mm.). In the anterior papillar}' plane and a little in front the 

 nasal-floor cartilage is well developed, but not of very great lateral 

 extent. The nasal septum comes well down and anteriorly the 

 nasal-floor cartilage abuts squarely against it; but in the middle 

 region of the papilla the septum has begun to retreat, and the 

 inner end of the nasal-floor cartilage curves up towards it some- 

 what. There is on each side a well developed septal ridge, and 

 the nasal-floor cartilage sends a feebly developed process towards 

 it. In PL XLi v. fig. 1 , the ridge process is not so well developed as 

 just in front. In this section will be seen a feature which is 

 developed in all the Diprotodonts as distinguished from the Poly- 

 protodonts, in the great lateral development of all the sti-uctures. 

 The inferior septal ridges project more, making the l^ase of the 

 septal region much broader; the nasal-floor cartilages are further 

 apart at their inner ends, and the palatine processes which are 

 developed in connection with Jacobson's cartilages are, in their 

 early development instead of closely together as in the Polypro- 

 todonts, widely apart. The naso-palatine canal passes obliquely 

 upwards and backwards, and opens into Jacobson's organ on 

 practically the same plane as that in which it becomes part of the 

 general nasal cavity. In PI. XLiv. flgs. 2 and 3, the nasal-floor is 

 found divided. Jacobson's cartilage is hollowed slightly on the 

 inner side, and in • the hollow lies the palatine process of 

 the premaxilla. In the region of best development Jacobson's 

 cartilage is present as a slightly concave plate, which 

 inclines markedly outwards as well as downwards from 

 the base of the septum. The palatine process is present as 

 a small ossified bar lying along the middle of the inner side. The 

 organ itself is almost oval on section; the inner wall of which is 



