The ossicula vary in shape in different parts of the skeleton, and Prof. E. Forbes affirms, that the careful determination of their characters is of great importance, since they are the only paints of the animals likely to be preserved, and the shape of an ossicle is as truly indicative of a genus or species, as is that of a bone among the vertebrata. There is one ossicle situated on the side of the centre of the disk, which is worthy of remark, because it often occurs in the chalk mingled with the debris of the rest of the skeleton, and should be preserved by the collector. It differs from all the other bones in being marked with radiating grooves, and is called the madreporiform tubercle; it appears to be the analogue of the stem of the Crinoideæ, in other words, a rudimental condition of an organ, which is fully developed in that order of radiata. [Sidenote: OPHIURA.] Ophiura. _Lign. 97, fig. 1._--Several species of this genus, which is distinguished by the long, slender, serpent-like arms, and the circular disk covered with plates and spines, have been found in a fossil state: one species was discovered in the Lower Silurian deposits by Prof. Sedgwick, and other forms have been obtained from all the succeeding formations. The Lias near Lyme Regis and Charmouth has yielded many beautiful examples of _Ophiura Egertoni_. Professor John Phillips has figured a species (_Oph. Milleri_, Geol. York, pl. xiii.) from the marlstone of Yorkshire, and a species from the Oxford Clay has been described as _Ophiura Prattii_. In the Cretaceous formation, remains of several species have been found. The first specimen from the Sussex Chalk that came under my notice, was discovered many years since, by my son, in a quarry at Preston, near Brighton; the rays were admirably preserved, as shown in the portion figured in _Lign. 97._ An example of this species, with the disk entire, and portions of five arms, was found by Henry Catt, Esq. and is represented in pl. xxiii. _fig._ 2, of Mr. Dixon's work.[KF] [292] Three plates are devoted to the Cretaceous Star-fishes: the descriptions by Prof. E. Forbes comprise twenty-five species, belonging to the genera Oreaster, Goniaster, Stellaster, Arthraster, and Ophiura, all from the Chalk of Sussex and Kent. [Illustration: Lign. 97. Fossil remains of Star-fishes. Fig. 1_a_.--Part of the ray of Ophiura serrata; _nat._ _Chalk, Preston._ (_Mr. Walter Mantell._) 1.--Portion of the same magnified. 2.--Goniaster Hunteri. _Chalk. Gravesend._ ] [Sidenote: GONIASTER--ASTERIAS.] Goniaster. _Lign._ 97 and 98.--The star-fishes of this genus, popularly called Cushion-stars, are of a pentagonal form, and have a double series of large marginal plates, bearing granules or spines; the latter are seldom preserved in the fossils. The upper surface is nodulose. [Illustration: Lign. 98. Fossil Star-fish. _Chalk, Kent._ Goniaster Mantelli. (_sp Forbes._[293])] [293] Dixon's Cret. Foss. p. 332.