260 THE AMERICAN NATURALIST [Vol. XLVII 



or substraction of a pair of mammae must occur at some 

 intermediate point. If a pair is added, it may be asso- 

 ciated with a closer spacing of the teats, or possibly with 

 a greater body length from the tip of the sternum to the 

 border of the groin. If subtracted, the opposite con- 

 dition would obtain. No observations were made on this 

 point, and indeed it would be difficult to make them, be- 

 cause of the elastic nature of the tissue, the difficulty in 

 handling the animals, and their marked variability in 

 size and proportions. 



There are two common sorts of asymmetry that may 

 occur separately or together. These are the ' ' suppressed 

 nipple" and the " triangular " patterns. The former 

 consists in the non-appearance of one member of a pair, 

 while the latter shows one teat placed opposite two teats 

 on the other side. The single teat is located at a point 

 midway between the opposite two, and is always on the 

 mammary line. The suppressed mamma variation ap- 

 peared in 29.8 per cent, of the offspring and the triangle 

 in 21.7 per cent. 



The suppressed mamma variation may occur twice in 

 the same animal, either on the same or opposite sides of 

 the body, the two sorts of repetition being about equally 

 frequent. But whenever the variation is repeated on the 

 same side of the body, a normally placed pair of mammae 

 always intervenes. Less frequently a pair of mamma? 

 seems to be omitted altogether, leaving an empty space 

 where normally a pair of mammae would occur. This may 

 be interpreted as the suppression of two mammae on op- 

 posite sides in one pair. The writer is rather doubtful 

 of this interpretation, however, and does not include it in 

 the 29.8 per cent, already mentioned. The omission of a 

 pair was found in onlv 5 cases, or less than 1 per cent, 

 of all. 



The triangle pattern shows three or perhaps four types 

 of compounding. Two triangles may appear with the 

 single nipples on the same side and separated by one or 

 more normal pairs, or they may be separated similarly, 



