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 
erouped 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. tibs with only capitulum and no tuberculum. 
Mammary glands of the sudoriparous and not the sebaceous 
type of epidermic gland. 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 
' Gegenbaur, Zur Kenntniss der Mammarorygane der Monotremen, Leipzig, 1886. 
