A. C. Coles 
41 
The parasites were exceedingly infrequent, and even in my prepara¬ 
tions I should not expect to find one in less than 15 to 20 minutes 
careful search. 
Most commonly they are situated somewhat outside the centre of 
the red corpuscle, and at times they are quite on the margin. (Plate II, 
fig. 19.) 
By far the commonest is a round or oval-shaped body, very faintly 
stained blue in the centre and surrounded by a denser ring, half of 
which is stained red, the other half blue. There is no pigment. 
Pear-shaped parasites were exceedingly rare, but dividing forms were 
sometimes met with. 
The nucleus is placed on the margin of the parasite and extends as 
a cap or segment -J- to ^ way round. Sometimes the chromatin is split 
up into two, three or four little masses which are nearly always placed on 
the margin. Minute points of chromatin, situated a little distance from 
the nucleus, could in some cases be detected, representing the centrosome 
of some writers, more probably extra-nuclear kai’yosomes. In some of 
the parasites on very careful examination a distinct fine line could be 
seen extending from the nucleus, across the body and terminating in a 
red stained spot on the other margin of the ring, just as was the case 
with those found in the field mouse. 
I found forms resembling a loaf of bread or figure of 8, as if budding 
had taken place. A few divided mesially, in the long axis, into two, 
each of which had a nucleus, whilst in others the two minute parasites 
were separating and were shaped like the letter V. 
I found two specimens in which division into four had taken place, 
in one the four young parasites were attached to one another, in the 
other four minute pear or oval-shaped parasites were arranged in the 
form of a cross. (Plate II, fig. 19 and Plate IV, fig. 45.) 
Generally speaking these parasites closely resemble those found in 
the field mouse, but they were on the whole a little smaller. 
The average diameter of the red corpuscle of the water vole I found 
to be about 5'9 p,, and the average size of the enclosed parasite varies 
from 1 to 2 yx, none apparently exceeding 2‘5 /x. in diameter. 
If this represents a new species I would suggest the name Nuttallia 
microti sp. n. ? 
