530 DR. P. CHALMERS MITCHELL ON THE 



Marsupials. The Tubulidentata appear to have a simplicity like that of the 

 Xenarthra, but still more like that of Marsupials. The Pholidota stand far removed 

 from any other Mammals, their modification of the primitive simplicity being peculiar 

 to the group and recalling the pattern in some birds. 



The Hyracoidea show a pattern which, except for one peculiarity, has moved little 

 from the primitive mammalian type. Meckel's tract is long and simple but for the 

 presence upon it of the accessory csecum. The hind-gut is relatively long and has no 

 very marked differentiation. The paired cseca are primitive. There is a total absence 

 of the special peculiarities found in the Ungulata or Rodentia. 



The Sirenia are almost diagrammatic in their primitive Mammalian simplicity. 

 Meckel's tract and the hind-gut are almost equal in length, and are suspended by a 

 practically continuous mesenteric fold. The two regions are marked off by the caeca, 

 which in one member of the group are paired. There is no trace of any of the 

 Ungulate specialisation. 



The Proboscidea are almost as primitive as the Manatidse, the intestinal tract 

 consisting of a Meckel's tract and a hind-gut not very different in length suspended on 

 a continuous mesentery. The duodenum, however, is more distinct, and there is a 

 single caecum. There is no sign of any special Ungulate character. 



In the Cetacea, the Odontoceti show a much elongated gut of great apparent 

 simplicity, no distinct regions being displayed. From the evidence afforded by the 

 Mystacoceti, however, it is plain that the simplicity of the Toothed Whales is secondary, 

 and that the Cetacean stock contains the ordinary mammalian pattern of the intestinal 

 canal in a simple form, from which no suggestion of affinity can be derived. 



In the Artiodactyla an interesting set of conditions is displayed. In all the sub- 

 groups of the order Meckel's tract is relatively long and simply arranged. The hind- 

 gut, however, is also long, being either as long as, or relatively longer than, in the 

 primitive mammalian type. It displays, however, a tendency to be modified in a 

 definite direction, peculiar to the Order. The caecum varies, but in a number of cases 

 shows evidence of a primitive paired arrangement. The special Artiodactyle peculiarity 

 consists in the tendency of the colic region to be greatly elongated in the form of what 

 is really a single loop, in the course of which minor loops may be developed. This 

 loop outgrows the primitive mesentery, and is frequently supported by special bands, 

 or is folded against, and attached secondarily to, the Meckelian mesentery. This colic 

 loop, moreover, tends to assume a spiral arrangement. 



In the Non-Uuminantia the colic loop is enormously long and capacious, its proximal 

 limb being especially large in calibre. It is arranged in a rude conical spiral, the tip 

 of the loop forming the apex of the spiral. 



In the Traguloidea the spiral coil is very short and of even calibre, consisting of not 

 more than two or three turns, which, moreover, are arranged in an irregular pyramid. 



