1. An account is given of a sporozoan parasite, hitherto known as Monocystis thalassemae Lankester, and its coccidian nature is demonstrated.
2. Schizogony and sporogony both occur in the eggs of Thalassema neptuni while these are in the genital pouches (nephridial sacs). Schizogony was observed on three occasions, and only the later stages were seen.
3. The full-grown sexual parasites are vermiform and immobile. The males are smaller than the females.
4. The microgametocyte, when it becomes spherical, gives rise to very numerous, elongate microgametes, similar to those of Aggregata.
5. Fertilization has not been observed. The oocystic membrane is only sometimes formed and is always inconspicuous.
6. At all stages where a chromosome count was possible, the number of chromosomes was seven. The reduction division is zygotic. No polymitotic figures were observed either in microgametogenesis or in sporogony.
7. Numerous sporocysts are produced. These may contain as many as twelve nuclei, and presumably give rise to at least twelve sporozoites. The formation of sporozoites has not been observed by us, but was noted by Lankester.
8. A new coccidian genus, Ovivora, is created for this organism, which now becomes Ovivora thalassemae (Lankester) and is relegated to the subfamily Aggregatinae in Hoare's classification.