304 PROFESSOR ROLLESTON ON THE 



torn away from the uterus." There are, however, it must be confessed, but few propo- 

 sitions which can be made of all unguiculate, or of all deciduate Mammalia beyond those 

 which the two names connote. These, however, by themselves are sufficient to justify 

 us in retaining the binary division of Professor Weber, coinciding as it does so nearly 

 with the class founded by Linnaeus on the peculiarities of a system so far removed from 

 the reproductive as is the tegumentary. And to them we may add the defenceless con- 

 dition in which the young of nearly all deciduate Mammalia, except the Elephant (and 

 Hyrax ?), are brought into the world, and the general, though not universal, coexistence 

 in them of multifid livers and multifid lungs with simple stomachs. The value of the 

 placentary system of classification is much better seen when we come to the subdivi- 

 sions of Weber's great class, when we see that of each of the well-established orders, 

 Simiadse, Insectivora, Rodentia, and Carnivora, a well-established and distinct aggre- 

 gation of placental characters can be predicated. Of the Chiroptera I do not speak, as 

 I have only been able to examine a single example from this order, and that but for 

 a short time and at a comparatively early period of development. Of the placentae of 

 the four other orders we may say, as perhaps of the entire sets of characters belonging 

 to each of the orders themselves, that those of the Carnivora are more distinctly 

 marked off from each and all of the other three than is any one of the other three from 

 any other of the three ; and of these three the Insectivora possess, on the whole, a 

 nearer affinity to the Simiadae than do the Rodents. In each case the characters are 

 those of the placenta at or near full time. 



In the SimiadcB we find the ultimate ramifications of the umbilical vessels confined to 

 the placenta ; and in it the fcetal capillaries are probably not merely apposed to simliar 

 maternal vessels, but plunged within a maternal sinus-system. The presence in them 

 of structures known as decidua reflexa and decidua vera show that all the aeration 

 and all the nourishment which the foetus receives comes from the single or double 

 placenta, and not at all from the extraplacental uterine mucous membrane. The deci- 

 duous serotina is ordinarily separable from the uterine surface of the placenta in a 

 coherent sheet, whilst in the substance of the placenta, besides other maternal elements, 

 there are the processes known as " Decidua-Fortsatze " mixed up inextricably like them, 

 but, unlike them, distinguishable by the naked eye. 



The Insectivora have the umbilical vessels confined to the placenta, but their ultimate 

 capillaries are apposed to similar maternal vessels not immersed in maternal sinuses. 

 Their decidua reflexa is more or less incomplete. The placental site may be anywhere 

 in the circumference of the uterine tube. The Tenrec's maternal and foetal structures 

 are exceedingly aberrant. The upgrowth of the chorion, the absence of any envelope 

 exteriorly to the amnios, and the distinction of the utero-placental region into two arese, 

 clothed with distinct kinds of mucous tissue, are unique points in this animal. By its 

 amniotic corpuscles it resembles the Elephant among deciduate, and several orders 

 amongst the non-deciduate mammals. 



