of the liodentia to the MarsupiaUa. 293 



cated in the Muriform animals. Even in the common domes- 

 tic mouse the division of the stomach into two halves, of 

 which that on the left has horny epithelium and that on the 

 right glandular mucous membrane, is very striking. In the 

 Hamster these divisions of the stomach are visible externally, 

 and in the field-mice with persistently growing molars, 

 which are the most specialized, we also find the greatest com- 

 plication in the structure of the stomach, as, indeed, has 

 already been fully described by Retzius. 



The Marsupials possess a true cloaca, and their lineal rela- 

 tions, the Rodents, agree with them pretty directly in this 

 respect. For the former possession of such an arrangement 

 is always manifested by the fact that the external orifices of 

 the urogenital apparatus and the anus are placed close 

 together, so that they nearly touch and are surrounded by 

 common sphincters. In a mature embryo of the beaver I 

 found them close together in a common naked and somewhat 

 sunken area. 



In Marsupials the two cornua of the uterus open by sepa- 

 rate apertures into the vagina ; in the Rodents the same con- 

 dition prevails, and its homological significance is not 

 destroyed by a short fusion of the two cornua in some few 

 Rodents. 



The greatest number of teats is attained in the Marsupialia, 

 Rodentia, and Insectivora. Taking into consideration the 

 circumstance that the occurrence of rudimentary teats in other 

 divisions of the Mammalia indicates reduction from a previous 

 more abundant endowment, the numerous teats of the Rodents 

 should indicate the primitive organization of those animals. 

 Moreover, Gegenbaur has shown that the milk-glands of the 

 Rodentia are in perfect homology with those of Marsupials. 



The structure of the larynx is directly connected with that 

 of the Marsupials, as already indicated by Mayer in 1829 ; 

 and R. Owen has long since stated that the brain of the 

 Rodentia agrees with that of the Marsupialia in essential 

 points. Not only the external form, but the internal struc- 

 ture is homologous in both. In common also there are the 

 poverty of convolutions, the want of a well-developed corpus 

 callosum, the strong development of the vermiform body in 

 the cerebellum, and the free position of the corpora qiLadrir 

 gemina. 



On the spinal cord the spinal nerves are arranged as in 

 Marsupials; the lumbar region especially, according to Jhering's 

 investigations, presents the greatest similarity. 



But what particularly confirms me in adhering to the 

 assertion that the Rodents are related to the Marsupials in a 



Ann. & Mag. N. Hist. Ser. 6. Vol. vi. 21 



