248 DR. p. CHALMERS MITCHELL ON THE' 



primitive type quite unlike the divei'gence found in any other 

 group, and support the supposition that the Cetacea diverged 

 from other Eutherians at a very remote period. 



I am not now prepared to associate the Perissodactyla and the 

 Artiodactyla on the evidence afibrded by gut-patterns. Both 

 groups may well have arisen independently from the common 

 stock. In all the Artiodactyles there has been a great lengthening 

 of the portion of the hind-gut formed from the distal limb of the 

 pendant loop. In Hijypojyotaimts this lengthening is irregular ; 

 in the Suidse it forms a definite spiral. In. the Traguloidea, 

 Tylopoda, and Pecora this spiral i-eappears but gradually be- 

 comes more specialized and more intimately associated with the 

 mesentery and blood-vessels of Meckel's tract. There may be 

 another expansion of the same region proximad of the spiral, 

 forming an ansa paraccecalis, and another distad of it forming a 

 second ansa coli dextra. 



In the Perissodactyla the whole of the recurrent limb of the 

 pendant loop distad of the large csocum gives rise to an enormous, 

 narrow, ansa coli dextra, an arrangement quite different from 

 that in any other group. It is certainly remarkable to find that 

 the herbivorous Perissodaotyles have developed a type of gut- 

 pattern extremely like that of the herbivorous Artiodactyles, 

 unless we are prepared to think that adaptation plays only a 

 secondary part in the matter. 



Among the Rodents we might expect to find convergent 

 resemblances with Artiodactyles, but these are quite superficial. 

 The mode in which the hind-gut is lengthened, the spiral twisting 

 of the cfecum, its relation to the paracfeoal loop, the frequent 

 doubling of the ansa coli dextra, and the frequent appearance 

 of an ansa, coli sinistra compose a general picture quite different 

 from that of the Artiodactyla or Perissodactyla. 80 far as gut- 

 pattern is concerned, the Rodents may well have ai'isen as a 

 distinct outgrowth of the primitive stock. 



The gut-patterns of Insectivora are consistent with the con- 

 ception that there has been a secondary reduction or simplification 

 Avithin the group from such a modification of the primitive 

 Mammalian type as is seen in Macroscelides, The pattern of 

 Macroscelides might belong to any simple Marsupial or Mono- 

 treme-like creature ; it diflers from the Marsupial patterns 

 most closely resembling it, by the presence of an ansa coli 

 dextra instead of an ansa coli sinistra, and from the Monotreme 

 pattern in having the cfecum some distance from the ajaex of 

 the pendant loop instead of very close to it. 



The examination of one of the Chiroptera in which the ofecum 

 is present has enabled me to distinguish between the very simple 

 patterns of Insectivora and of Chiroptera. In Chiroptera the 

 whole of the pendant loop becomes Meckel's tract, and the hind- 

 gut is reduced to an extremely short and straight i-ectum. No 

 doubt the bix'd-like shortening of the hind-gut is a secondary 



