THE ARCHENOEPHALA. 97 



On the other hand, the sub-class Gyrencephala receives the 

 following definition : — 



" The third leading modification of the Mammalian cerebrum 

 is such an increase in its relative size, that it extends over more 

 or less of the cerebellum, and generally more or less over the 

 olfactory lobes. - Save in very few exceptional cases of the 

 smaller and inferior forms of the Quadrumana, the superficies 

 are folded into more or less numerous gyri, or convolutions, 

 whence the name Gyrencephala, which I propose for the third 

 sub-class of Mammalia" — L. c, p. 18. 



I am quite unable to see what these so-called definitions de- 

 fine. If, for example, we place the brains of an Ant-eater, or of 

 a Capybara, side by side with that of a Genett — the two former 

 being Lissencephala, the latter one of the Gyrencephala — either 

 " definition ' will apply equally well to either of the three 

 brains. All three have slightly convoluted brains ; in all three 

 the olfactory lobes and cerebellum are more or less uncovered ; 

 and nothing in the definitions of the sub-classes of this " cerebral 

 classification " would enable an anatomist to say that any one of 

 these three brains belonged to one sub-class rather than another. 



Since Mr. Waterhouse pointed out the fact, no one has 

 doubted that, as a general rule, the brains of the so-called " Gyren- 

 cephala" are more convoluted, size for size, than those of the 

 " Lissencephala ;" and the relations of the size and the zoological 

 position of an animal to the characters of its cerebral surface 

 have long since been well discussed by Gratiolet, Dareste, and 

 others. But it is exactly because the rule is only a general 

 one, and has many exceptions, that the degree of cerebral convo- 

 lution must be rejected as the basis of the definition of any 

 large group of Mammals. 



Thus far, we meet, in Professor Owen's definitions, with a 

 certain foundation in fact, though it may not be such as is fitted 

 to afford ground for classification : but the group " Archencephala" 

 is in a more unfortunate position. Our first canon comes into 

 operation, and we must reject it, because the statements respect- 

 ing matters of fact in its definition are untrue. The words stand 



thus : — 



H 



