-1go1-1902.] The Entomostraca of Mid-Lothian. 307 
any specimen. The shell is usually so opaque, and the body 
and limbs so completely enclosed in it, that little or nothing 
of the internal structure can be seen. The opacity of the 
shell also renders it difficult to see the characteristic external 
_ markings, which are present in many species; but this may be - 
done by careful adjustment of the light. We have, first, the 
so-called “lucid spots ”, which are really the points of attach- 
ment of certain muscles; the shape and number of these spots 
in each species is fairly constant and distinctive. Secondly, 
_ there is also in some species a well developed pattern on the 
shell. By putting a specimen into a solution of caustic potash 
we render the shell semi-transparent, and can see the general 
shape of the limbs; but this process effaces the shell-markings, 
and should therefore only be done after they have been 
observed. 
. It is extremely difficult to dissect the ostracods, and after a 
few experiments we gave up the attempt. In order, therefore, 
to identify our species, we had to pay careful attention to the 
size, colour, and outlines of the shell. If we were given a 
specimen of either Cypris fuscata or C. incongruens, and asked 
_ to say which of the two it was, we should find it difficult to 
; decide the point at first sight; but if we were to draw the 
t outline accurately with the help of the camera lucida, that 
alone would be sufficient to settle the question. Cypris 
incongruens narrows very slightly towards the front, and C. 
fuscata towards the back; and, so far as we have observed, 
this small difference is always to be relied on. 
We chanced, while still quite inexperienced as regards the 
ostracods, to get a number of Cypris virens and C. incongruens, 
both young and old, in a single collection; and since the very 
young Cypris virens differs considerably in outline from the 
adult, and the half-grown forms of C. virens and C. incongruens 
are somewhat alike, we could not tell for some time how many 
species we really had—whether 1, 2, or 3. 
Being so well protected by their shells, and their habit of 
concealing themselves in mud, the ostracods do not need to 
have their powers of locomotion well developed. Some of 
them are very active, but all the species we know are quite 
easy to catch at once with a dipper, whereas you may require 
to chase a copepod patiently for a minute or two before 
