1 1 6 THE OPOSSUMS. 



wards ; the second resembles it in general form, but 

 is larger and broader ; the third is small, and re- 

 sembles the tuberculous molars of the true Camivora. 

 In the lower jaw the three real molars do not 

 materially differ in point of size. They are narrower 

 than those of the upper, have their tubercles arranged 

 in a single longitudinal series, a single large one in 

 the centre, and a smaller on each side. 



" The Yapock has very large cheek -pouches, which 

 extend far back into the mouth, and of which the 

 opening is very apparent." 



It does not appear, from the above description, that 

 the dentition of the Yapock is " altogether different" 

 from that of the Opossums. This animal agrees with 

 Didelphys (and with no other mammiferous animal) 

 in the number of its incisors, and as regards the other 

 teeth, the only important difference consists in the 

 number of molars. The absence of a false molar on 

 each side of the upper and under jaws would appear 

 of little importance, inasmuch as such a deficiency is 

 frequently observed in nearly allied animals. The 

 permanent absence of a true molar is much more 

 worthy of consideration. These differences have not 

 yet been discovered in an adult animal, for Dr. Nat- 

 terer, who brought three specimens with him from 

 South America, informs me that the animal ex- 

 amined by Mr. Ogilby was young, and that an adult 

 specimen which he possesses, is much larger. Two 

 of M. Natterer's specimens,, he informs me, were 

 caught near water not far distant from Rio Janeiro ; 

 the third was captured alive, near Para, in a basket 

 to those used in this country for catching 



