(7/a.«s 6. Mammalia. Order 1. Monotremnfa. 495 



only produce one or quite a few offspring at a birth ; whilst in 

 small Mammalia tlie time is short, and several or many young ones 

 are born at the same time. If there are several embryos in the uterus 

 at once, the ova from which they develop have all been fertilised at 

 the same time ; they are, therefore, all at about the same stage of 

 development, and are born in immediate succession. In some 

 placental forms the offspring at birth are very helpless, naked, 

 with closed eyes (the eyelids adhering together) ; whilst others are 

 more advanced, or are immediately capable of independent movement. 

 They are all at first nourished by milk from the parent. 

 The Mammalia are grouped in the following way; — 



A. Oviparous Mammalia. Egg laid, large, sm-rounded by a shell. 

 Cloaca long. One order only, the Monotremes. 



A. Aplacental Mammalia. Egg small, develops in the oviduct, whose 

 walls secrete a fluid for the nourishment of the embryo. This is very small and 

 imperfectly developed at birth. Cloaca rudimentary (only present in the 

 female). One order only, the Marsupials. 



C. Placental Mammals. The embryo develops from a small egg : the 

 outer embryonic membrane is closely connected by villi with the wall of the 

 oviduct. Cloaca absent. All other orders of Mammalia belong here. 



Order 1. Monotremata {Monotremes). 



This small group differs from other Mammalia in a number of 

 characteristics which bring it near the Eeptilia. It is especially 

 remarkable that Monotremes are oviparous; that the egg is 

 relatively large, and is surrounded by a leathery shell; that 

 they possess a well-developed cloaca. Other points tending in 

 the same direction, are the presence of well-developed cervical 

 ribs, a large coracoid, a very reptilian e pi sternum, the 

 absence of a scapular spine, the form of the stapes, the 

 straight cochlea, the presence of cartilage in the sclerotic of 

 the eye, the feeble development of the corpus callosum, the 

 whole relations of urinary and genital organs, as already 

 noticed. It is interesting that the temperature of the body* 

 is lower than in other Mammals. 



That they are not incorrectly placed with the Mammalia may be 

 seen, however, from the fact that they accord with them, and differ from 

 Reptiles in the following respects : they are covered with hair, possess 

 sebaceous and sudoriparous glands, have a long, jointed sternum, 

 no quadrate, two occipital condyles, three ear bones; whilst 

 the mid-brain is divided into four lobes, the penis is tubular, etc. 



It may also be mentioned that Monotremes possess marsupial 

 bones, like those of Marsupials, connected with the pelvis ; further, 



* In Echnidna under iTsual conditions 28° C, in Ornithorhynchy,s 2g ° Q. (c). In 

 pther Mammalia as a rule 38 — 39 " C. 



