I.] MONODELPHIA. 7 



of their cerebral, dental, and in many cases of their repro- 

 ductive organs show an inferior grade of organisation to thai 

 of the generality of the sub-class. The next order, about 

 the limits of which there is no difficulty, is the Sirenia 

 aquatic vegetable-eating animals, with complete absence of 

 hind limbs, and low cerebral organisation represented in our 

 present state of knowledge by but two existing genera, the 

 Dugongs and Manatees, and by a few extinct forms which, 

 though approaching a more generalised Mammalian type, 

 show no special characters allying them to any of the other 

 orders. 



Another equally well marked and equally isolated, though 

 far more numerously represented and diversified, order is 

 that of the Cetacca, composed of the various forms of 

 Whales, Dolphins, and Porpoises. In aquatic habits, 

 external fish-like form and absence of hind limbs, they 

 resemble the last, though in all other characters they are 

 as widely removed from them as from any other order 

 among the Monodelphia. The association by systematists 

 of the Cetacea and Sirenia in our group can only be made 

 either in ignorance of their true structure, or in an avowedly 

 artificial system. 



All the remaining orders are more nearly allied together, 

 the steps by which they have become modified from one 

 general type being, in most cases, not difficult to realise. 

 Their dentition especially, however diversified in detail* 

 always responds to some modification of the well-known 

 formula incisors %, canines |, premolars *, molars . Al- 

 though the existing forms are broken up into groups, in 

 most cases easy of definition, the discoveries already made 

 in palaeontology have in great measure filled up the gaps 

 between them. 



Very isolated among existing Monodelphia are the two 



