The external surface of the umbrella is divided into two regions, an aboral region and a marginal region, by a well-marked circular groove (the coronal groove). The aboral region is usually smooth and undivided, but it is an elongated dome, {322}thimble- or cone-shaped, in marked contrast to the flattened umbrella of the Discophora. The margin is divided into a number of triangular or rounded lobes, and these are continued as far as the coronal groove as distinct areas delimited by shallow grooves on the surface of the umbrella. The tentacles arise from the grooves between the marginal areas, and are provided with expanded bases called the pedalia. The manubrium may be short or moderately long, but it is never provided with long lips. FAM. 1. PERIPHYLLIDAE.[358]—Coronata with four or six statorhabs. In _Pericolpa_ (Kerguelen) there are only four tentacles and four statorhabs. In _Periphylla_, a remarkable deep-sea genus from 700 to 2000 fathoms in all seas, but occasionally found at the surface, there are twelve tentacles and four statorhabs. The specimens from deep water have a characteristic dark red-brown or violet-brown colour. They are usually small Medusae, but the umbrella of _P. regina_ is over 21 cm. in diameter. _Atorella_ has six tentacles and six statorhabs. FAM. 2. EPHYROPSIDAE.—Coronata with eight or more than eight statorhabs. _Nausithoe punctata_ is a small, transparent jelly-fish, not exceeding 10 mm. in diameter, of world-wide distribution. Its Scyphistoma stage is described on p. 317. _N. rubra_, a species of a reddish colour found at a considerable depth in the South Atlantic and Indian Oceans, is probably an abysmal form. _Palephyra_ differs from _Nausithoe_ in having elongated instead of rounded gonads. _Linantha_ and _Linuche_ differ from the others in having subdivided marginal lobes. FAM. 3. ATOLLIDAE.—_Atolla_ is a deep-sea jelly-fish of very wide geographical distribution. It is characterised by the multiplication of the marginal appendages, but the number is very irregular. There may be double or quadruple the usual number of marginal lobes, or an indefinite number. There may be sixteen to thirty-two statorhabs, and the number of tentacles is quite irregular. Some of the species attain a considerable size, the diameter of the umbrella of _A. gigantea_ being 150 mm., of _A. valdiviae_ sometimes 130 mm., and of _A. bairdi_ 110 mm. {323}ORDER IV. DISCOPHORA. This order contains not only by far the greater number of the species of Scyphozoa, but those of the largest size, and all those that are familiar to the seaside visitor and the mariner under the general term jelly-fish.