INTESTINAL TEACT OF MAMMALS. 533 



types. In many of the New World Monkeys this simple hind-gut has degenerated, 

 becoming shorter. In the Old "World types the primitively undifferentiated simple 

 loops of the hind-gut tend to appear as the definite structures known to human 

 anatomists as the very large loop composed of ascending, transverse, and descending- 

 colon, and the irregular kinks known as the sigmoid flexure. A characteristic feature 

 of this group, which does not appear in any marked form in the Prosimise, is the 

 elaboration of the connection between the colic region and the mesentery of Meckel's 

 tract. 



If, then, we were to arrange the existing Mammalia solely on the evidence to be 

 derived from the characters considered in this Memoir, we should recognise an ancestral 

 group the members of which would be associated merely because of their possession ot 

 an ancestral type of intestinal tract, and for the separation of which consideration 

 of other characters would be necessary. This group would contain : — 



Marsupialia. 



Xexarthka. 



TuBULIDENTATA. 



Galeopithectd^e. 



From this primitive centre eleven radiating streams have been formed with only in 

 one or two cases any indications of the relative position of the streams. Each stream 

 sets out from the primitive centre and proceeds in a definite direction. 



C Htracoidea. 



1. j SlREXIA. 



I Peoboscidea. 

 This first stream has moved very little from the primitive type, and its members are 

 associated rather by common absence of differentiation than by peculiarities. 



2. MOXOTREMATA. 



Xot far removed from the centre, but along a definite radius. 



3. PlIOLIDOTA. 



The group has moved far, and in an absolutely definite way. 



4. Cetacea. 



More complete information wanted, but, so far as known, the group has moved alonti' 

 a definite radius. 



0. U.VGL'LATA. 



These, again, are on a very definite radius, the most specialised members having moved 

 very far from the centre. The Perissodactyla first branched off from the radius pear 

 the centre; next the Non-Ruminamia have separated, and further out the Traguloidea, 

 and, finally, towards the furthest point of the radius the Tylopoda and l'ecora. 



vol. xvn. — part v. No. 13. — December, 1905. 4 a 



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