Zooecia cylindrical, their furrowed keel never prominent _diffusa_, p. 223. beta'. Zooecia (or at any rate some of the zooecia) constricted or tapering at the base, their emargination and furrow conspicuous _allmani_, p. 224. II. Ectocyst stiff; zooecia truncated when the polypide is retracted. Surface of zooecia minutely roughened, distinctly annulate on the distal part _tanganyikæ_, p. 225. III. Ectocyst swollen and contractile, capable of transverse wrinkling all over the zooecium; zooecia never emarginate _punctata_, p. 227. [Footnote BF: In specimens preserved in spirit they are apt to collapse and therefore to become somewhat concave.] There has always been much difficulty in separating the species of _Plumatella_, and even now there is no general consensus of opinion as to the number that should be recognized. The difficulty, however, is much reduced if the following precautions are observed:-- (1) If the zoarium appears to be tangled, if the branches intertwine or overlap, or if the zooecia are closely pressed together, the whole mass should be carefully dissected out. This is necessary not only because zoaria belonging to different species are sometimes found entangled together but also because it is often difficult to recognize the characteristic method of branching and shape of the zooecia unless it is done. (2) As large a part as possible of each zoarium should be examined, preferably with a binocular microscope, and allowance should be made for irregularities and abnormalities of all kinds. What must be observed is the rule rather than the exceptions. (3) When the statoblasts are being examined, care must be taken that they lie flat and that their surface is parallel to that of the nose-piece of the microscope. If they are viewed obliquely it is impossible to see their true outlines and proportions. (4) In order to see the relative proportions of the capsule and the swim-ring it is necessary that the statoblast should be rendered transparent. This is often difficult owing to the presence of air in the air-cells, but strong nitric acid applied judiciously will render it possible (p. 240).