482 DR. P. CHALMERS MITCHELL ON THE 



Group Castoroidea. Castor fiber. 



In the Beaver, one example of which I have examined, the general pattern is similar 

 to that of Xerus. The duodenum consists of a number of loops not well separated 

 from the anterior part of Meckel's tract. The latter region is almost identical with 

 that in Xerus. The caecum is very capacious. The portion proximad of the entrance 

 of the small intestine is extremely long and tapers to a point. Posteriorly, it passes 

 into a much-sacculated expanded portion, the line of which is continuous with a 

 sacculated, much-dilated anterior portion of the hind-gut. The general configuration 

 of this region strongly suggests that the existing caecum is one member of an original 

 pair. The colic region of the hind-gut consists of two loops, the distad of which is 

 very long and irregular, bearing several minor loops, the Avhole being folded over the 

 Meckelian mesentery. The rectal region is relatively short and nearly straight. 



The general arrangement of the veins is similar to that in Xerus. 



Groups Geomyoidea, Anomaluroidea, and Myoxoidea. 

 I have not examined any of these. Flower (5) states that the caecum is absent in 

 Muscardiwus avellanarius, a condition recalling the Insectivora and obviously secondary 

 in Rodents. 



Group Dipodoidea. Alactaga indica, Dipus cegyptius. 



In the Jerboas, of which I have examined several specimens, the pattern is similar 

 to that in other Rodents. The duodenum consists of a number of short loops not well 

 marked off from Meckel's tract. The latter is suspended by an elongated oval expanse 

 of mesentery and is relatively rather short and wide, being thrown into numerous very 

 short, regularly disposed loops, and ending in a straight recurrent portion. The caecum 

 is not so capacious as in the Squirrels and Beavers, but seems to vary considerably in 

 different individuals. It is long, and gradually tapers to a point. The hind-gut 

 consists of a colic region formed of three very definite loops, folded over the Meckelian 

 region. The rectum is relatively long and straight. The anterior mesenteric vein 

 has the normal course. It is joined by a very large caecal vein, and by three veins 

 from the three colic groups, and at the edge of the mesentery unites with the posterior 

 mesenteric vein which drains the rectum. 



Group Myoidea. Musdecumanus (fig. 27), Mas arboricola, Cricetus frumentarius. 



In the Rat the duodenum is a well-marked separate loop. Meckel's tract is relatively 

 long and is composed of a number of long, irregular, minor loops forming a contorted 

 mass at the periphery of a semicircular expanse of mesentery, and terminating in a 

 straight, recurrent portion. The caecum is capacious, tapers to a blunt point, and has 

 a well-marked spiral curvature. The hind-gut is relatively short and is divided into a 

 colic loop which lies loosely folded over the Meckelian mesentery and is generally 



