106 PROTOTHERIA MONOTREMATA CHAP. 



the latter are only known from very fragmentary remains, 

 which are not sufficient to determine the systematic position of 

 the animals of which they are fragments, I have not thought 

 it worth while to attempt a serious definition of the order 

 Multituberculata. I have introduced a short account of the 

 principal facts which are known concerning the creatures 

 grouped together under this name into the historical sketch of 

 the progress of mammalian life in Chapter IV. As to the 

 Monotremata, there is no question that they are entitled to rank 

 in a group equivalent to that including all other mammals of 

 which we have sufficient knowledge to construct a classificatory 

 scheme. There have been, indeed, naturalists, such as Meckel, 

 who would altogether deny the mammalian rank of these 

 creatures. 



The Monotremata or Ornithodelphia may be thus defined : 



Mammalia with no teats, but with a temporary pouch in 

 which the young are hatched, or to which they are transferred 

 after hatching, and into which open the ducts of the mammary 

 glands. An anterior abdominal vein, or at least the membrane 

 supporting it, persists throughout the abdominal cavity. Heart 

 with an incomplete and largely fleshy right auriculo-ventricular 

 valve. Brain without a corpus callosum. Shoulder girdle with 

 a large coracoid reaching the sternum ; clavicles and an inter- 

 clavicle present. There are " marsupial " or epipubic bones 

 attached to the pelvis. Vertebrae with no epiphyses for the 

 most part. Ribs with only capitulum and no tuberculum. 

 Mammary glands of the sudoriparous and not the sebaceous 

 type of epidermic gland. 1 Oviparous, with a large-yolked and 

 meroblastic ovum, enclosed within a follicle of two rows of cells. 



To call these animals Mammalia is of course an abuse of the 

 meaning of that word in one sense, but it is not in another ; 

 since the pouch of these Monotremes is, as has been explained 

 elsewhere (p. 16), the real equivalent of a teat, and not of the 

 pouch of the Marsupials. 



The most salient characteristic of this group of mammals in 

 the estimation of their position in the vertebrate series is not 

 so much the fact that they are oviparous as that the eggs are 

 large-yolked, and develop therefore, so far as regards their early 

 stages, after the fashion of the egg of a reptile. The laying 

 1 Gegenbaur, Zv,r Kenntniss der Mammarorgane der Monotremen, Leipzig, 1886. 



