OF THE CLASS MAMMALIA. 17 



The third leading modification of the Mammalian cerebrum is 

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



of form, that there are two distinct discoid placentae in Callithrix as in 

 Cercopithecus, Macacus and Semnopithecus ; whilst in Mycetes, as in Troglo- 

 dytes, there is but one such placenta. 



The structure of the discoid placenta in the Pteropus, like that of the Eat, 

 more resembles that of the foetal portion of the cotyledon in the Cow than that 

 of the ceUulo-vascular spongy placenta of the Quadrumana ; and this difference, 

 with the more important one of the larger umbilical sac, appears to me to 

 greatly outweigh the degree of resemblance in mere outward form of the placenta. 

 Any argument in favour of the affinity of the Cheiroptera to the Quadrumana, 

 based on that degree of resemblance, must be affected by the prevalence of the 

 double discoid placenta in the Quadrumana. Since Hunter first ma.de known 

 that modification* in a species of Macacus, which, from a comparison of tho 

 foetus now preserved in the Museum of the Royal College of Surgeons, I 

 behove to be the ' Wrinkled Baboon ' of Shaw {Macacus rhesus, Desm.), 

 Professor Breschet has described and figured the two separate discoid placentae 

 in the small South American Squirrel-monkey (Callithrix sciureus, Kuhl), in 

 the Green Monkey {Cercopithecus sabceus, Desm.), and in the Long-nosed 

 Monkey (Semnopithecus nasicus). Yet this well-marked modification of the 

 cellulo-vascular placenta is not constant in the Quadrumana, or even in the 

 primary groups of the order. In the Platyrhines, e.g., the Howler (Mycetes 

 seniculus, Kuhl) ha3 a single placenta, and amongst the Catarliines, I have 

 ascertained that, in the Chimpanzee (Troglodytes niger) the placenta is single, 

 as in the Human subject. 



The five flat placental lobes, virtually as distinct as if they were separate 

 placenta?, in the Hare, resemble more the subdivided placentae of the Sloth than 

 the single hemispheroid pedunculate placenta of the Rat, or the flattened circular 

 placenta of the Howler Monkey. In short, the observed differences of form 

 in the placentae of the Rodentia, Insectivora, Cheiroptera and Quadrumana by 

 no means justify the use of one general term as applicable to the wholef. 



The second argument for the association of the Insectivora, Cheiroptera and 

 Rodentia with the Quadrumana is taken from alleged conformity of cerebral 

 structure. 



" Le cerveau d'un Rongeur differe a peine de celui d'un Insectivore ; il existe 

 aussi une ressemblance extreme entre l'encephale d'un Insectivore et celui de 

 certains Quadrumanes ;" whence it is meant to be inferred, that there is the same 

 extreme resemblance between the brain in Rodentia and certain Quadrumana. In 

 my paper on the ' Brains of the Marsupialia ' (Phil. Trans. 1837), I hare described 

 and figured (pi. v. p. 93) the brain of a Beaver (see fig. 2, p. 15) and that of 

 a small Monkey (Midas rufimanus, fig. 3, p. 19), showing the absence of cere- 

 bral convolutions in both. As the cerebral hemispheres have since been shown 

 to be equally smooth in other Hapalidce of Isidore Geoffroy, in the Potto Lemur % 

 (Perodicticus, Bennett), in Microcebus§, and with few and feeble traces of con- 



* Animal Economy, 4to. 1780. 



f Annales des Sciences Nat. torn. cit. p. 96. 



X Bijdrage tot de Kennis van den Potto van Bosnian, 4to. 1851, V. der 

 Hoeven. 



§ Comptes Rendus de l'Acad. des Sciences, Janvier 19, 1852. 

 LINN. PROC. — ZOOLOGY. 2 



