XVI PREFACE. 



class now ceases to be an exception to the rest of the vertebrates 

 in this respect. The primordial kidneys have been demonstrated 

 in the early period of the fish-embryo by Reichert. See his 

 paper TJeber die Mueller- Wolff' schen Korper hei Fischembryonen in 

 Mueller's ArcMv, 1856, pp. 125—133. 



It is to be much regretted that the original and highly valuable 

 memoir of Prof. Owen On the characters, prmciples of division, 

 and jpri:mary groups of the class Mammalia published in the 

 Proceedings of the Linnean Society for 1857, should have made its 

 appearance at a period too recent to admit of that general discus- 

 sion of it by Zoologists, which in the opinion of the author of this 

 work ought to precede the adoption of any great change in the 

 systematic arrangement. The memoir contains in its preliminary 

 portion a condensed view of the author's various determinations 

 respecting the dental characters of the class. It will be found that 

 full use has been made of these in the characters of the genera in 

 the present volume. 



In this memoir the fourfold primary division of the mammalian 

 class is founded upon the four leading modifications of the cerebral 

 system. In some mammals the cerebral hemispheres are only 

 partially connected by the fornix and the anterior commissure ; in 

 the rest of the class the corpus callosum is added. 



Those in which the corpus callosum is absent are the impla- 

 cental or marsupial mammals. This first group includes the 

 Lyencephala, so called from the comparatively loose and discon- 

 nected state of the cerebral hemispheres. The hemispheres, again, 

 are so small that the olfactory ganglions and cerebellum are ex- 

 posed, and more or less also of the optic lobes. The surface of the 

 hemispheres is generally smooth. 



In the next stage of development of the brain the corpus cal- 

 losum is present, but connects hemispheres not exceeding those of 

 the former class in bulk and external character; there are no convo- 

 lutions, except in a small number of the largest members of the group, 

 where they are simple and few. In this subclass the testes are 

 either permanently or temporarily concealed in the abdomen; there 

 is a common genito-urinary aperture in most; there are two anterior 

 vencB cavce terminating in the right auricle of the heart, with other 

 remarkable indications of resemblance to birds and reptiles. These 

 are named Lissencephala. 



