INTESTINAL TEACT OF MAMMALS. 531 



Distad of this, the hind-gut displays a second peculiarity which I have described as 

 the supra-Meckelian fold. 



In the Tylopoda the spiral coil is enormous, of even calibre, and arranged as a flat 

 watchspring. The supra-Meckelian fold is well developed. 



In the Pecora the spiral coil is extremely long, very narrow in calibre, and the flat 

 watchspring-] ike disposition is typical. The supra-Meckelian fold is equally typically 

 developed. Additional extensions of the hind-gut appear irregularly as loops proximad 

 of the spiral coil, or distad of the supra-Meckelian fold in the rectal region. 



The Artiodactyle colic region, then, displays a definite case of an anatomical 

 peculiarity, so well-marked and complex as to be a safe guide to affinity. This 

 consists in the elongation and spiral coiling of the proximal portion of the colon. In 

 the Non-Ruminantia it has developed in one direction ; in the Traguloidea it has 

 remained in the primitive Artiodactyle stage. In the Tylopoda and Pecora it has 

 developed in a characteristic direction different from that in the Non-Ruminantia. 

 The evidence given by the supra-Meckelian fold, however, Avhich is common to the 

 Traguloidea, Tylopoda, and Pecora, would seem to associate these three more closely, 

 to the exclusion of the Non-Ruminantia. 



In the Perissodactyla, as in the Artiodactyla, the characteristic differentiation is in 

 the hind-gut. That region is long and very capacious, and is marked by the enormous 

 development of the colic loop. In all three, the latter is a very long, narrow, simple 

 loop, the proximal and distal limbs of which are closely approximated and are of huge 

 calibre. There is perhaps a remote resemblance with the Non-Ruminant Artiodactyles, 

 but in the Perissodactyles there is no trace of the minor expansions of the colic loop 

 or of the spiral coiling which is the characteristic feature of the Artiodactyle 

 arrangement. The Tapirs, Ehinoceroses, and Horses, far apart as they are in many 

 ways, have retained an identical mode of adaptation to vegetable diet, thus showing a 

 dominance of the inherited element over the purely adaptive element. The Artio- 

 dactyla and Perissodactyla have in common little more than that in both the 

 primitive pattern of the hind-gut has been modified by adaptation to a herbivorous 

 diet. This modification is essentially an elongation and specialisation of the colic 

 region, but the mode of the elongation differs in the two groups. 



The Rodentia have moved away from the primitive mammalian type in a definite and 

 characteristic fashion. Meckel's tract tends to be elongated, but retains a comparatively 

 simple arrangement. There are abundant traces of a primitive double condition of the 

 caeca. The hind-gut has increased in length and this increase tends to assume the 

 form of a pair of long colic loops, which in some of the larger forms are extremely 

 complicated in the adult, and which in some of the smaller forms are partly merged 

 in one another. The characteristic peculiarity, however, is that the normal caecum, 

 which usually becomes very capacious, tends to be spirally coiled, the coiling involving 



