Prof. J. D. Dana on the Classification of Mammals. 211 



The other Mammals are either true viviparous species, or semi- 

 oviparous. 



The latter, including the Marsupials and Monotremes, con- 

 stitute a natural group, as usually so regarded, the most funda- 

 mental characteristic of which — the immaturity of the young at 

 birth, by which they are related to oviparous Vertebrates — 

 suggests the name Ooticoids. 



The viviparous species are variously arranged by diflferent 

 zoologists*. Prof. Owen, basing his subdivisions largely, as has 

 been stated, on the characters of the brain, makes the two groups 

 Gyrencephala and Lissencephala, the former so named from 

 having, in general, the surface of the brain convoluted, and the 

 latter from its being, with some exceptions, smooth. 



The Gyrencephala include, in Prof. Owen's system, three 

 groups : — I. the Unguiculata (consisting, as presented by him, 

 of the orders 1, Quadrumana, 2, Carnivora) ; II. the Ungulata 

 (1, Artiodactyla or Ruminantia ; 2, Perissodactyla or Solidungu- 

 lata and Multungulata, 3, Proboscidia, 4, Toxodontia) ; III. the 

 Mutilata (1, Sirenia, 2, Cetacea). The Lissencephala comprise 

 four orders, arranged by him as follows : (1) Bruta or Edentata 

 (Sloth, &c.), (2) Cheiroptera or Bats, (3) Insectivora (Mole, 

 Hedgehog, &c.), (4) Rodentia. 



Although the characteristics of the brain do not set forth 

 satisfactorily the distinctions between the Gyrencephala and 

 Lissencephala, the groups themselves (first laid down with the 

 limits here assigned, as Prof. Owen states, by Jourdan) appear 

 to be founded in nature. In the arrangement of the gi-uups 

 under these two divisions, however, the system proposed below 

 widely differs from the above. 



The Crustaceans have here also afforded the writer the prin- 

 ciples of classification on which he rests his conclusionsf. 



The orders among Crustaceans are based not only on a dif- 

 ference of structure and cephalization, but also on a difference 



* See Professor Owen's memoir already referred to, for an account of 

 diflPerent earlier systems of the classification of Mammals. 



t Principles are none the less important because indicated among these 

 lower Articulates. The turns of a closed spiral are easily mistaken for 

 circles, as was long the case with those of flowers in plants ; but if the 

 spire be drawn out long, it then exhibits its true characters, and may dis- 

 play details that are otherwise undiscoverable. The class of Crustaceans 

 is an example of a type of structure thus drawn out, its species ranging 

 from the microscopic memberless Rotifer to the highest crabs ; and the 

 genera are distributed, so to speak, at distant intervals along the course of 

 the series, since they are comparatively few in number. Fundamental 

 principles in zoological science are therefore exhibited in this class on a 

 magnified scale, easily perceived and understood. 



