Nucleolites (_Wond._ p. 328).--There is a small type belonging to this family, of which several species are so abundant in the Oolite, Greensand, and Chalk-marl, that a brief notice of their characters may be useful. The shell is oblong and inflated, rounded in front and flat behind. The pores are united by grooves; the outlet is in a deep furrow on the superior face; the mouth is sub-central. One species occurs in the Tertiary strata, and there is a recent species inhabits the seas of Australia. There are coloured figures of Nucleolites in _Pict. Atlas_, pl. liv. fig. 5, pl. lv. figs. 6, 8. Spatangidæ.--In this tribe of echinites, the case is oblong or cordiform. The mouth, elongated transversely and destitute of proper jaws, is situated in front of the centre of the base, near the anterior border of the periphery. The outlet is towards the posterior margin. The tubercles and spines are very small. Four subdivisions are established, namely,-- 1. _Ananchytes_.--A thick and oval shell; the ambulacra simple and converging towards the summit; the mouth transverse; the outlet is situated on the inferior face. (_Lign. 104, fig. 2._) 2. _Spatangus._--This name is now restricted to those Spatangidæ in which the ambulacra are petaloid, the external row of pores slightly elongated, and the inner rows round. 3. _Micraster._--By this term are now distinguished the Spatangidæ which have the ambulacra depressed, and the shell cordiform. The pores of the even ambulacra are united by a furrow. The vent is on the posterior face. The common chalk Spatangus belongs to this genus. (_Lign. 107._) 4. _Holaster._--The shells are heart-shaped; the ambulacra simple, and converging towards the summit; the mouth is transverse; the outlet is within the posterior face. [Sidenote: ANANCHYTES OVATUS.] Ananchytes ovatus. _Lign. 104. fig. 2._--These sea urchins are among the most characteristic of the fossils of the Upper Chalk, and are peculiar to the Cretaceous formation. They are readily distinguished by their elevated helmet-like form, and by the transverse mouth and oblong outlet situated on the inferior face of the flat base, and towards the margin. (_Lign. 104. fig. 2^a._) The vernacular names of "_Shepherd's Crown_," and "_Fairy Loaf_," indicate the form of these abundant fossils. The shell is oval in its longest diameter; flat, or nearly so, below; and rounded, conical, and somewhat laterally compressed towards the summit. The ambulacra are five, between double lines of pores; the tubercles are minute and scattered; the substance of the shell is of great thickness. More than twenty species of the genus are known.