The principal genera are _Monograptus_, with the axis straight, curved, or helicoid, from many horizons in the Silurian strata; _Rastrites_, with a spirally coiled axis, Silurian; _Didymograptus_, Ordovician; and _Coenograptus_, Ordovician. FAM. 2. DIPRIONIDAE.—Cups arranged in two or four vertical rows on the axis. _Diplograptus_, Ordovician and Silurian; _Climacograptus_, Ordovician and Silurian; and _Phyllograptus_, in which the axis and cups are arranged in such a manner that they resemble an ovate leaf. FAM. 3. RETIOLITIDAE.—Cups arranged biserially on a reticulate axis. _Retiolites_, Ordovician and Silurian; _Stomatograptus_, _Retiograptus_, and _Glossograptus_, Ordovician. FOSSIL CORALS POSSIBLY ALLIED TO HYDROZOA. Among the many fossil corals that are usually classified with the Hydrozoa the genus _Porosphaera_ is of interest as it is often supposed to be related to _Millepora_. It consists of globular masses about 10-20 mm. in diameter occurring in the Upper Cretaceous strata. In the centre there is usually a foreign body around which the coral was formed by concentric encrusting growth. Running radially from pores on the surface to the centre, there are numerous tubules which have a certain general resemblance to the pore-tubes of _Millepora_. The monomorphic {283}character of these tubes, their very minute size, the absence of ampullae, and the general texture of the corallum, are characters which separate this fossil very distinctly from any recent Hydroid corals. _Porosphaera_, therefore, was probably not a Hydrozoon, and certainly not related to the recent _Millepora_. Closely related to _Porosphaera_ apparently are other globular, ellipsoidal, or fusiform corals from various strata, such as _Loftusia_ from the Eocene of Persia, _Parkeria_ from the Cambridge Greensand, and _Heterastridium_ from the Alpine Trias. In the last named there is apparently a dimorphism of the radial tubes. Allied to these genera, again, but occurring in the form of thick, concentric, calcareous lamellae, are the genera _Ellipsactinia_ and _Sphaeractinia_ from the Upper Jurassic. Another important series of fossil corals is that of the family STROMATOPORIDAE. These fossils are found in great beds of immense extent in many of the Palaeozoic rocks, and must have played an important part in the geological processes of that period. They consist of a series of calcareous lamellae, separated by considerable intervals, encrusting foreign bodies of various kinds. Sometimes they are flat and plate-like, sometimes globular or nodular in form. The lamellae are in some cases perforated by tabulate, vertical, or radial pores, but in many others these pores are absent. The zoological position of the Stromatoporidae is very uncertain, but there is not at present any very conclusive evidence that they are Hydrozoa.