244 DR. p. CHALMERS MITCHELL ON THE 



facts do not seem to justify too close an identification between 

 the specialized loops in one mammal and another. I propose, 

 however, to give a tentative summary of the conditions. 



In Monotremes there is an ansa dextra near the distal end of 

 the pendant loop, and the i-ectum is straight. 



In Marsupials the rectum is relatively long and may be thrown 

 into minor loops. In the Polyprotoclonts there are no other 

 expansions of the hind-gut. In the Diprotodonts the usual 

 condition is the presence of a complex ansa sinistra, and there 

 may be in addition, as in the Phascolarctidfe, an equally complex 

 a7isa dextra. 



In the Edentata the rectum is always relatively long ; in the 

 Pholidota there is no further expansion. In the Tubulidentata 

 and Xenarthra there is also an ansa sinistra. 



In the Hyracoidea, Sirenia, and Proboscidea the rectum is 

 relatively long, especially in the region just distad of the pendant 

 loop, and therefore forming an ansa sinistra. 



In the Cetacea the rectum is straight, and thei'e is no ansa. 

 Among the Ungulata. vera, the I'ectum is always longer than 

 the distance between the distal end of the pendant loop aiid the 

 anus, and in the majority of the sub-groups the lengthening- 

 is most marked proximally, although, perhaps, not enough 

 specialized to be regarded as corresponding with an a^isa 

 sinistra. An ansa jxiraccecalis or postccecalis is present, just 

 distad of the cfecum ; in most of the Pecora, absent in the others, 

 but its presence, in addition to the well-known colic spiral, makes 

 it impossible to identify the latter with the paracaecal loop. 

 The recurrent limb of the pendant loop always forms at least 

 one large ansa dextra ; this is complex in the Hip]jo2)otamus, 

 and forms a spiral in the Swine, Traguloidea, Tylopoda, and 

 Pecora, and a very long narrow loop in the Perissodactyla. 

 In the Traguloidea, Tylopoda,, and Pecora there is a second 

 more distally placed ansa dextra, folded closely against Meckel's 

 tract between the colic spiral and the minor loops of the tract. 



In the Rodentia there is almost invariably a paracfecal loop 

 often spirally twisted, with the caecum or independently of it, 

 always at least one and frequently two ansce dextrce, which may 

 be straight, or spirally twisted, together or independently, and 

 pressed against Meckel's ti'act. An ansa sinistra is frequently 

 present, either as a, definite narrow loop, or as a complex loop, 

 and the latter condition grades oflT into a wavy condition of the 

 rectum, which in all Rodents is longer than the distance it has 

 to traverse. 



In the Insectivora the rectum is short and straight, but a 

 definite ansa dextra is usxially developed. 



In the Chiroptera the whole hind-gut forms a short straight 

 rectum, and there are no ansce. 



In the Carnivora the rectum, although relatively short, is 

 usually longei- than the distance it has to traveise (between the 

 distal end of the pendant loop and the anus), and veiy often 



