I860.] Cerebral System of the Class Mammalia. 177 



meiit of the hemispheres ; which are exteriorly smooth, or with few 

 convolutions, and not extended backward over the cerebellum. 



The third is that in which the cerebral hemispheres show, with the 

 corpus callosum, an increased size, extending backward more or less 

 upon the cerebellum, and, save in a few cases in which the size of the 

 mammal is small, having the exterior surface convoluted. 



The fourth modification exhibits, with all the preceding complica- 

 tions, a marked and sudden augmentation in the relative and absolute 

 size of the cerebral hemispheres, which now extend above and beyond 

 the cerebellum, and have a posterior lobe, with a posterior horn of the 

 lateral ventricle, and a * hippocampus minor,' and which have the 

 gray exterior matter increased by numerous and deep convolutions. 



The mammals exemplifying the lowest types of brain, were called 

 Lyencephala,* from the comparatively loose or disconnected state of 

 the cerebral hemispheres : they are unguiculate : some have the ' optic 

 lobes ' simple, others partly subdivided ; the lobes being then called 

 ' bigeminal bodies.' 



The Lyencephala with simple optic lobes are * edentulous' or 

 without calcified teeth, and are devoid of external ears, scrotum, 

 nipples, and marsupial pouch : they are true ' testiconda : * they have 

 a coracoid bone extending from the scapula to the sternum, and also 

 an epicoracoid and episternum as in Lizards : they are unguiculate 

 and pentadactyle, with a supplementary tarsal bone supporting a 

 perforated spur in the male. The order so characterized is called 

 * MoNOTREMATA,' in reference to their single excretory and generative 

 outlet. It includes two genera — Echidna and Ornithorhynchus. 

 Of the first, the species are terrestrial, insectivorous, chiefly myrmeco- 

 phagous, having the beak-like slender jaws and long cylindrical tongue 

 of the true anteaters ; but they are covered, like the hedgehog, with 

 spines. Of the second genus, the species are aquatic, with a flattened 

 beak, like that of a duck, which is used in the anserine manner to 

 extract insects and worms from the mud : but they are clothed with 

 a close fine fur like that of a mole, whence the name ' duck-mole ' by 

 which these anomalous quadrupeds are commonly known to the 

 colonists. Both genera of Monotremes are strictly limited to Australia 

 and Tasmania. 



'The Lyencephala with divided optic lobes, forming the * corpora 

 bigemina * and ' quadrigemina ' of anthropotomists, have teeth, and with 

 rare exceptions, the three kinds, viz. incisors, canines, and molars. 

 They are called the Marsupialia, because they are distinguished 

 by a peculiar pouch, which, in the female, contains the nipples and 

 shelters the young for a certain period after their birth : both sexes 

 have the marsupial bones in common with the JNfonotremes ; a 

 much varied denition, especially as regards the number of incisors, 

 but usually including four true molars ; and never more than three 

 premolars : the angle of the lower jaw is more or less inverted. 



* Kvw, to loose ; hyY.4<pa\05^ brain. 



