1908.] ANTECHINOMYS AND OTHER MARSUPIALS. 581 



mammals. The paired ceeca of Hyrax have no mesenteries of any 

 kind attached to them. They hang perfectly free of peritoneum 

 folds into the body-cavity. I do not think that any true unpaired 

 caecum, even the small one of the Carnivora, is thus free of 

 mesentery. 



The only other Mammals known to m.e in which the ceecum 

 or caeca are thus entirely free of peritoneal folds, are the American 

 Edentates. On the other hand, the unpaired caecum of Hyrax 

 has peritoneal folds connected with it. These are not mentioned 

 by Dr. Mitchell, and they have a very important bearing upon the 

 natiu^e of the unpaired caecum of Hyrax. It will be seen from the 

 accompanying figvire (text-fig. 115, p. 582), which may be com- 

 pared with that given by Dr. Mitchell to which reference has 

 already been made, that the colon where it leaves the caecum is 

 bent sharply upon itself, and then forms a more or less L-shaped 

 curve. The whole of this part of the colon is attached to the 

 caecum by a mesentery which runs to the very tip of the caecum. 

 When the cfecum is examined more closely, the caeco-colic ligament 

 is seen to arise from the caecum along a straight line distant from, 

 but continuing the line of, the ileum where it enters the caecum. 

 This fixed loop of the colon is surely to be compared to the single 

 colic loop of the Perissodactyles, where indeed, as I myself * and 

 others have pointed out, the same ligament occurs but is much 

 shorter. The slightly twisted commencement of this ansa para- 

 caecalis, as I interpret it, is reminiscent of that of many Artio- 

 dactyles and Rodents (e. g. Arvicanthis : for which see below, 

 p. 589), but the length of the entire ansa is obviously more like 

 that of the Perissodactyles. This marked loop is not figured by 

 Dr. Mitchell, who only represents an alteration in the direction 

 of the " small intestine " (as he regards it) where it leaves the 

 impaired caecum. There is no mention in the text of any such 

 ansa coli ; the author to whom I am referring contenting himself 

 with remarking that " the hind gut is subdivided into a distinct 

 colic loop (C.L.) and a long straight rectum." The " loop " in 

 question is not comparable to the ansa coli described by myself 

 in this paper, and by others. It is merely the wavy transverse 

 colon in which there are as a rule no ansae at all. It is regrettable 

 that Dr. Mitchell has used indifferently the same term (" colic 

 loop ") for the fixed ansae coli which are permanent structui-es, 

 and for alterations in the direction of the gut. The colon of 

 Hyrax in fact, after leaving the ansa paraca^calis refei^red to, has 

 no further anste coli. It has an irregularly looped course owing 

 to its great length, and passes upwards giving off the paired caeca 

 in an ascending colon, then runs across the body-cavity as the 

 transverse colon, and descends in a series of wavy convolutions 

 as the descending colon into the rectum. As is the case in all 

 of the more specialised Mammalia, the omentum is attached 

 to the transvei'se colon. And the mode of its attachment is 



* " The Anatomy of the American Taph-," P. Z. S. 1889, p. 254. 



