The Chalcidoid wasp, Bairamlia fuscipes, was found parasitising the flea Ceratophyllus wickhami in the nest of the North American Grey Squirrel (Sciurus carolinensis) from Gerrard's Cross.

The larval and pupal stages of B. fuscipes were found in the cocoons of C. wickhami, and the adults were crawling about in the squirrel's nest.

The parasites were scarce, only 7 per cent, of the cocoons dissected at intervals during fifteen months were parasitised.

The parasite has not been reared in the laboratory beyond the second instar larva, though full grown larvae have successfully reached maturity under experimental conditions.

B. fuscipes oviposits through the flea cocoon and fixes the egg on the abdomen of a resting larva or pupa. Only one egg has been found in each parasitised cocoon. The larva feeds externally on the flea, fixing itself to the fourth abdominal segment or thereabouts.

Attempts were made to parasitise larvae of Xenopsylla astia, but without success.

The external anatomy of the full grown larva of B. fuscipes is described.

1. The main facts in Brasil's (1907) account of the intracellular schizogony in Selenidium caulleryi from Protula tubularia have been confirmed. Early stages in the sporogony of this species are described now for the first time.

2. Intracellular schizogony is described for the first time in Selenidium mesnili Brasil from Myxicola infundibulum, and here also the early stages in sporogony are noted for the first time.

3. The life histories of Caullery and Mesnil's two unnamed species of Selenidium (now called S. spionis (Köll.) and S. foliatum n.sp.) from Scolelepis fuliginosa have been exhaustively studied. No schizogonic phase has been observed in either. The gametocysts, here described for the first time, are found to develop fully only after escape into the sea. The gametocysts of S. spionis are ovoid, 108μ x 60μ and contain spores with four sporozoites: those of S. foliatum are spherical, 70μ in diameter, and their spores contain eight sporozoites.

4. New or hitherto little known species of Selenidium are recorded from Cirratulus cirratus Müll., Branchiomma vesiculosum Mont., Sabella pavonina Sav., Pomatoceros triqueter L. and Terebella lepidoria Kühl, and the morphological characters of the trophozoites are described.

5. A discussion follows of the value of the diagnostic characters of the genus Selenidium. It is suggested that this genus requires drastic revision and will probably have to be dismembered. Stress is laid, however, on the occurrence in all the gregarines examined, and at all the observed stages of their development, of characteristic chromatic bodies at the anterior end, structures which have hitherto escaped the notice of most observers.