27?. The American Naturalist. [April, 
were found in one horn. They formed ovoidal swellings of the 
uterine cornu separated from each other by a slight interval. 
They were nearly an inch long and not quite three-fourths of 
an inch in diameter. Upon carefully inserting the point of a 
scissors through the uterine wall ventrally, and opening it so 
as to expose the embryo in its membranes, it was found that 
the latter were not adherent to the mucosa, except over a 
small quadrate area on the mesometric side. After the pla- 
centa was forcibly detached with part of its decidua, the scar 
left on the uterine wall measured 9 mm. in length over its short 
diameter which coincides with the direction of the passage in 
the cornu. Its diameter the other way or transversely to the 
uterine cornu was 12 mm. The edges of the scar forming its 
short diameter were slightly elevated so as to form a pair of 
slight folds projecting above the non-placental area above and 
below the embryo. These folds represent a very rudimentary 
decidua reflexa, traces of which are also present in forms with 
a zonary placentation. The edges of the scar forming the 
margins of its long diameter pass gradually into the mucous 
membrane of the uterine walls, and there is no such well- 
marked fold representing a reflexa as appears on the other 
sides. The peculiar quadrate form of the placenta was equally 
manifest in its foetal part, or that to which the umbilical cord 
and membranes are attached. The area of the placenta in 
millimetres is, in round numbers, 9x12 = 108 sq. mm. Over 
all the remaining portions the foetal membranes were not at- 
tached to the uterine mucosa. There was a strongly developed 
decidua vera over the placental area. 
If we now compare this peculiar quadrate placenta with the 
ordinary zonary type the homologies of its parts will become 
clear, and^ think it affords demonstrative evidence of the direct 
derivation of this quadrate form from one which was zonary. 
If, for example, we select the zonary type, as seen in the cat of 
the third or fourth week of intra-uterine life, and mark off a 
quadrate portion of the placental girdle which will be as 9 is to 
12, 9 being the width of the girdle and 12 the proportional 
length of a segment of it measured along its curve, we shall 
have a placenta which is the morphological equivalent of that 
seen in the red squirrel. 
In the rabbit's uterus of the eighth day of gestation there is 
a proliferation or thickening of the dorsal or mesometric side 
of the uterine wall, which betrays distinct traces of a squarish 
figure. As this represents the site of the future placenta in the 
rabbit it is plain that the squirrel has retained in a far more 
