362 SYSTEMATIC ZOOLOGY 



fetus, — there is never a direct continuity of the cavities of the 

 maternal and fetal vessels. This organ of nutrition, consisting 

 in part of portions of the fetal membranes and in part of por- 

 tions of the uterine wall, is termed the placenta, and is found in 

 various forms in most mammals. In some the membranes 

 separate readily from the wall of the uterus at birth ; in others 

 the connection is so intimate that a part of the uterine lining is 

 torn away at birth and forms the decidua or afterbirth. The 

 period of development or gestation, as it is technically termed, 

 varies much, from three weeks in some mice to twenty months 

 in the elephant ; there is also much variation in the number of 

 young that may be born at one time. A few mammals are 

 monogamous, but this is rare. 



While the majority of the Mammalia are land animals, some 

 live most of the time in the air and others are wholly or partly 

 aquatic. Many of them are gregarious; in cold countries many 

 pass into a winter sleep ; they then take no food, the tempera- 

 ture of the body is lowered, and the heart beat and respiration 

 are slower. One of the most striking characteristics of this 

 class is found in the care taken of the young, which are gen- 

 erally born in a more or less helpless condition, and are fed and 

 cared for by the parent until they are able to care for themselves. 



Fossil remains of the Mammalia are numerous, and many 

 extinct species were of gigantic proportions. Concerning the 

 origin of the group there is considerable doubt ; through fossils 

 there is a distinct relationship to the Reptilia, and through cer- 

 tain anatomical and embryological characteristics a relationship 

 with the Amphibia may readily be established. 



SUBCLASS I. ORNITHODELPHIA, OR MONOTREMATA 



The Ornithodelphia (dr. opvis, bird, and 8e\<f>v<;, womb), or 

 Monotremata (Gr. pdvo^, single, and rptjpa, opening), also called 

 the* Prototheria (Gr. irpwros, first, and Oi-jplov, beast), constitute 

 a small group of mammals found to-day only in Australia and 

 the adjacent islands of Tasmania and New Guinea ; fossil re- 

 mains, however, have been found in America and Europe. They 

 differ from all other Mammalia in having a cloaca and conse- 

 quently a single cloacal opening to the outside instead of dis- 



