206 Proceedings of the Biological Society of Washington. 



foetuses of Neotoma and Mus. Parsons (Proc. Zool. Soc. Lond., 

 1898, p. 878) notes that the alimentary canal of foetal Pedetes 

 caffer corresponds very accurately with that of its mother. 



Mus mil sen I us L. 



All our material has been collected in Boulder. The maximum length 

 of the small intestine is 49.8 cm., of the large 15; these measurements 

 occurred in the same individual. About 40 cm. for the small and 9 for 

 the large is more usual. The following figures represent the ratio of the 

 large to the small intestine in the examples measured; the length of the 

 small intestine is taken as 100, and that of the large is therefore expressed 

 in percentages of it, decimals omitted. The numbers in parentheses refer 

 to the number of individuals under each category. 15(1); 17(1); 18(2); 

 19(3); 20(2); 21(2); 22(8); 23(2); 25(2); 26(1); 27 (1); 30(3); 

 3L (1 ) ; 33 (2) ; 84 ( 1 ). These figures suggest that larger numbers might 

 give a distinctly bimodal curve. 



In a female with ratio 100: 34, a foetus showed 100: 30. The following 

 figures show the actual measurements of all the specimens having the 

 small intestine over -10 or under 80 cm. ; the first figure is that of the 

 small, the second that of the large intestine. It certainly appears that 

 the smaller mice have the large intestine proportionately somewhat 

 longer (thus, the mice having the large intestine 30% or over the length 

 of small, have small intestine of the following lengths, 28.9, 49.8, 39.4, 

 40.2, 83, 28, 28), but there is no regular correspondence. 45.2-8; 42.8-8; 

 49.8-15; 44-8.5; 45.5-8.8; 44.5-9; 42-9; 49.6-7; 48-9.5; 40.2-11.2; 40.2- 

 12; 40.8-8.S. 28.9-9.9; 28-8.5; 28-9. 



Epimys norvegicus (Erxl.) 

 All collected in Boulder. The ratio of the small to the large intestine 

 is given in the same manner as above under M us. 10 (1) ; 11 (1) ; 12 (1) ; 



13 (5); 14 (2); 15 (2); 16 (2); IS (2). Thus it appears that Epimys 

 norvegicus differs conspicuously from Mus musculus in the proportions, 

 although the extremes overlap; the series is also more compact, without 

 any clear suggestion of a secondary mode. 



The actual measurements (in cm.) are given in the same manner as 

 those for Mus, for the individuals having the small intestine under 90 and 

 over 100 cm.; 126-23; 125-8.5; 136-18; 114-13; 119-18; 111-14; 101.2- 

 14; 103-17. S; 109-14.5; 104-14; 111.5-17; 72-12.0; 69-10.5. The last, 

 giving the smallest measurements, is a male. 



A well-marked example of the hooded rat, caught in Boulder, had a 

 much longer large intestine than any of the normal rats, the measure- 

 ments being 131-30. Is this racial, or an individual peculiarity? 

 Peromyscus boylii rowleyi (Allen). 



Collected by E. R. Warren at Irwin's Ranch, Los Animas Co., Colorado. 

 Ratios (given as under Mus): 58(1); 59(1); 60 (1); 64 (1); 07(1); 

 69 (1). The small intestine varies from 21.8 to 25.5 cm. ; the large from 



14 to 15.2. There is thus little variation in the series, hut probably 

 material from other parts of the animal's range would show more. The 

 proportions are entirely different from those of Mus and Epimys. 



