Marsupialia

The precise arrangement of the lateral and median vaginal canals varies widely in different individuals and species of the opossum family. In some, the lateral canals are completely separate and there is hardly a beginning of the median cul-de-sac ; in other species, however, it is very well developed and in still others transitional conditions are found. The attachment of the em bryo to the wall of the uterus is by the yolk-sack, there being no true placenta.

Some of the smaller species of opossums have a large number of young at a birth, together with a large number of teats (rarely, as many as 27) and a reduction or loss of the pouch. It is very doubtful, however, whether these are primitive characters; they seem more like retrogressive specializations within the family.

The common opossum (Didelphys virginiana) may reach a total length of 37 inches, the head and body being 22 inches long, while at the other extreme, in the shrew-opossum of Brazil (Peramys sorex) the length of the head and body is less than 3 inches. The murine opossum (Marmosa) of Central America and Brazil in cludes mouse-like insectivorous forms in which the pouch is ab sent. The water opossum or yapok (Chironectes) of Panama and Guiana is the only known marsupial adapted to a partly aquatic life. The toes of the hind feet are webbed and it resembles an otter in habits. The pouch is present.

The Australian Dasyuroids.

Under the name dasyuroids we may a series of Australian forms ranging from the tiny "pouched mice" to the wolf-like Thylacinus. They are frequently referred to collectively as "carnivorous marsupials." The ordi nary "pouched mice" of the genus Phascogale are extremely active little animals with the blood-thirsty habits of shrews. They live mostly in the bush or forest. They differ profoundly from true mice not only in their internal anatomy but especially in their dentition, which is of the carnivorous-insectivorous type. In his excellent work The Mammals of South Australia F. Wood Jones writes: "The Yellow-footed Pouched Mouse is an animal of great interest from a zoological point of view, since in the whole of its anatomy it shows itself to be a remarkably generalized ani mal. It represents a marsupial base form, its general anatomy being but little modified from a basal mammalian plan, and it stereotypes the simple creature that could be considered ancestral to most of the marsupial radiations." Although too specialized in

certain details, such as the reduction of the first toe of the hind foot and the loss of the caecum of the intestine, to be the direct ancestors of the diprotodont or herbivorous marsupials, the pouched mice of the genus Phascogale are at least structurally near to the diverging lines leading to the jumping pouched mice, to the dasyures, to the Tasmanian devil and other peculiar forms.

In the fat-tailed pouched mouse (Sminthopsis crassicaudata) the hind foot is much elongated and this feature is carried to an extreme in the jerboa pouched mouse (Antechinomys), which shows a maximum adaptation to the jumping habit, including a complete loss of the first digit of the hind foot. As in many other desert-living animals, the ears are very large and the bony shells covering the chambers of the middle ears are greatly enlarged. The animal is insectivorous and probably leaps after insects.

One of the most remarkable members of the pouched mouse group is the mulgara or crest-tailed pouched mouse (Dasycercus cristicauda) of South Australia. This sturdily-built, short-limbed little animal is a typical desert form, which has multiplied in an astonishing manner during the passage of a mouse plague across cultivated districts and is a fearless predatory animal.

The carnivorous adaptations of the mulgara are carried further in the so-called native cats or dasyures, which are about as large as true cats but have pointed muzzles. The blades on the upper molars are relatively longer and more obliquely directed than in the mulgara and the third upper premolar is now completely eliminated. The skull bears heavy crests and well developed cheek arches for the support of the powerful muscles. The general form of body is like that of a marten. In the spotted-tailed native cat (Dasyurus maculatus) the first digit of the hind foot is re tained and the foot pads are striated in conformity with its tree climbing habits; but in the common native cat (Dasyurus viver rinus) the first digit of the hind foot is lost and the foot pads are granular. It formerly inhabited treeless rocky country as well as cultivated districts and was a fearless predatory animal.