52 TRANSACTIONS OF THE [DeC. 12 



can alone explain such conditions as the identity of the dentition 

 in Cjnoidea and Arctoidea, whereas in another part of the same 

 organic system we meet with the transition from the compara- 

 tively highly developed and complicated cfficum of the former to 

 the simple ileo-colic junction ol the latter. In respect to .spe- 

 cific peculiarities of the condition as found in Procyon lofor, a 

 few facts deserve mention. 



The transition from large to small intestine is marked by a 

 difference in the calibre as well as in the structure of the wall of 

 the tube. For a distance of about 15cm. before the ileo-colic 

 junction is reached, the distal portion of the ileum pi-esents a 

 series of irregularities in calibre. A number (2-3) of constric- 

 tions at a few cm. interval occupy this terminal portion. They 

 are associated with an increase in the circular muscular fibres of 

 the gut, which appear somewhat aggregated in these situations, 

 but the narrowing of the lumen remains after complete relaxa- 

 tion and distension of the intestine. 



It appears reasonable to bring this condition into relation 

 with the complete absence of an ileo-colic valve. Some form of 

 mechanical separation of the lumen of the canal between large 

 and small intestine seems to belong to forms in which there is 

 an absence of caecum and ileo-colic valve. 



In 3Iani)i macrura this appears to be accomplished by the 

 abrupt bend of the tube at the point of transition and may serve 

 to render a return of contents from large to small intestine more 

 difficult. 



In Procyon the constrictions and the accompanying muscular 

 ai'rangements in the terminal part of the small intestine may 

 perfoi-m the same function. As regards the colon, a thicker 

 circular band of muscular fibres at the ileo-colic junction itself, 

 undoubtedly acts in the same way, although it does not impress 

 itself on the lumen of the canal as a jDermanent fold in the 

 i-elaxed and distended condition. 



An increased bearing of the convex margin of the large in- 

 testine may fairly be taken to represent the ceecal disposition. 



Several lines of investigation appear to promise results in this 

 connection. 



The embryological differentiation of large and small intestine 

 in Procyonid.X' may afford valuable information. It seems 

 probable that the ileo-colic junction in this form represents the 

 highest grade of the process of reduction and diminiition ob- 

 served in so many instances, that of our own ca3cum and vermi- 

 form appendix being the most marked. 



If such is the case the early stages of development should give 

 indications of the previous primitive form. 



