l6 THE ESSEX NATURALIST.
they may be linked in some at present unknown way with other
characters which are useful.
Fig. 2.
A. Mandible and Palp of a Living Young Fairy-Shrimp (Chirocephalus
diaphanus), at about the sixth free-swimming stage, x 250.
B. The Same Organs of a Young Stage of the Fossil Lepidocaris
rhyniensis (Middle Devonian), x 500.
The last problem to which I will refer is that of the persistence
in geological time of particular details of organic structure. As
is well known, certain animal genera, and perhaps even species,