196 LLOYD'S NATURAL HISTORY. 
wing-cases of Beetles. In their large number of incisor teeth | 
—five pairs in the upper, and four in the lower jaw—the 
Opossums, as shown in the figure on page 1209, differ from all 
other Mammals, the Pervamelide coming next to them in this 
respect, with, at most, five pairs in the upper, and three in the 
lower jaw. These teeth are arranged in each jaw nearly in the 
form of a semicircle; in the upper jaw the inner pair are 
somewhat longer than the rest, from the nearest of which they 
are generally separated by a narrow interval, their form being 
nearly cylindrical, with a slight dilitation at the summit. The 
canine teeth are well-developed, those of the upper jaw being 
somewhat longer than those opposed to them. 
In general form and appearance the majority of Opossums | 
may be compared to the common Grey Rat, except that the | 
muzzle is more elongated, and completely naked at the extremity, 
where it is perforated by the nostrils. In the larger species the 
form of the body is proportionately stouter. A peculiarity of 
the Opossums is that the teats, which are always numerous, 
varying from five to twenty-five, are always an odd number; 
either a single odd one, or as many as five being placed in the 
centre of a circle or oval formed by the others. 
In the case of the short-tailed group, forming the sub-genus 
Peramys, the series of specimens available to the author of the 
British Museum Catalogue of Marsupials was insufficient to 
allow of the proper determination of all the species, which may 
consequently stand in need of revision. 
THE TRUE OPOSSUMS. GENUS DIDELPHYS. 
Didelphys, Linn., Syst. Nat., ed. 10, vol. i., p. 54 (1760). 
I. COMMON OPOSSUM. DIDELPHYS MARSUPIALIS. 
Didelphys marsupialis, Linn., Syst. Nat., ed. 10, vol. i, p. 54 
(1760); Thomas, Cat. Marsup. Brit. Mus. p. 323 (1888). 
