1. Certain flocks in the Aberystwyth Area having suffered considerably from Liver Rot, a Survey of the Agricultural Zoology was carried out between October, 1913, and October, 1916, special attention being given to the above disease and the bionomics of the host snail L. truncatula.

2. The Area comprised some 250 sq. miles of North Cardiganshire, etc., including Plynlymon, and falls into six natural divisions.

3. The presence of the disease has caused a number of local modifications in farm practice, designed to prevent, or mitigate losses. The disease was found to be on the wane within the Area, though endemic in certain places.

4. L. truncatula and L. peregra are the only abundant local species of the Genus. Considerable differences occur in the distribution of these two species. L. peregra was only studied in order to better understand the habits of L. truncatula. This latter species occurs in shallow water floored with Diatomaceous clayey mud, and is absent from soft mud, and peat. There is a marked coincidence between the distribution of clay and L. truncatula, and L. truncatula and Liver Rot.

5. The growth of the snails takes place throughout the year unless interrupted by drought or severe frost. Ova are deposited in March–April and hatch in May. The resulting snails, when they become adults, measure about 5 mm. in length. They in turn deposit ova during June–July. Drought usually limits further progress until the late Autumn, the snail being often very scarce during August–September.

6. Desiccation speedily kills L. truncatula. Local environmental conditions—rainfall, aspect, soil, vegetation, etc.—play a very important part in respect to its survival. Rainfall and its distribution through the year is the most important factor governing the increase, spread, and infectivity of the snail.

7. The ova masses dry rapidly and then resemble a hard inconspicuous scale. They resume their former size and shape when moisture returns, and many appear to be able to hatch subsequently, though the amount of desiccation they will withstand is not yet determined. In spite of prolonged droughts, L. truncatula re-appears, unless ditches, etc., have been cleaned out.

8. Roadside ditches are very frequently inhabited by infected snails and the drainage from them may be the cause of cases of rot.

9. Cercariae of D. hepaticum were obtained from L. truncatula in large numbers practically throughout the year; but not from snails measuring less than 4·5 mm. in length. Seventy cercariae were given off naturally, within 48 hours, from one specimen measuring 7 mm. in length. Heavily parasitized snails are adversely affected. A few cercariae, resembling those of D. hepaticum were obtained once from L. peregra.