Their principal distinctive characters are these. The brain has a very large anterior commissure, and a very small corpus callosum, agreeing exactly in this respect with the Marsupials. The cerebral hemispheres, in _Echidna_ at least, are well developed and convoluted on the surface. The auditory ossicles present a low grade of development, the malleus being very large, the incus small, and the stapes columelliform. The coracoid bone is complete, and articulates with the sternum, and there is a pre-coracoid (epi-coracoid) in advance of the coracoid, while there is also a large “interclavicle” or episternum in front of the sternum, and connecting it with the clavicles. There are also “epipubic” bones. The oviducts (not differentiated into uterine and Fallopian portions) are completely distinct, and open, as in oviparous vertebrates, separately into a cloacal chamber, and there is no distinct vagina. The testes of the male are abdominal in position throughout life, and the vasa deferentia open into the cloaca, not into a distinct urethral passage. The penis, attached to the ventral wall of the cloaca, is perforated by a canal in the greater part of its length, and not merely grooved, as in reptiles and those birds which have such an organ. The canal is open at the base and brought only temporarily in contact with the termination of the vasa deferentia, so as to form a seminal urethra when required; but it never transmits the urinary secretion. This condition is a distinct advance on that of the Sauropsida in the direction of the more complex development of these parts in most of the other Mammalia. The ureters do not open into the bladder, but behind it into the dorsal wall of the genito-urinary passage. The mammary glands have no distinct nipple, but pour out their secretion through numerous apertures situated in a cup-shaped depression of the abdominal skin, forming a mammary marsupium, especially developed in the females during lactation. It should be mentioned that, according to the observations of Professor Gegenbaur, the mammary glands of the Monotremes are the simplest found in the entire class. The region of the glands is, indeed, distinguished from the rest of the abdomen merely by its thicker layers of muscles. The glands themselves are closely connected with the hair-follicles, and belong to the sudoriparous type, whereas the glands of all other mammals are of sebaceous origin. The young are produced from eggs laid by the female parent, which are meroblastic, like those of birds; that is to say only a portion of the yolk segments and forms the embryo, the remainder serving for the nourishment of the latter. The above are the principal distinguishing characters of the group, and apply not only to the subclass, but of course equally to the one order Monotremata, in which the two existing genera are included. In addition to these more important characters, the following minor features may also be mentioned.