Toxaster (_Spatangus_) complanatus. _Lign. 108._--In this form of Spatangus (constituting the genus _Toxaster Agass_.), the ambulacra are not depressed or furrowed, as in the preceding echinites, nor petaloid (_leaf-shaped_), as in those which M. Agassiz denominates true Spatangi, but converge to a point on the summit, as is shown in _fig._ 2; the external rows of pores are elongated horizontally, and form a kind of furrow. The odd ambulacrum is in a deep groove. The mouth is transverse, _fig._ 3, _d_; and at the anterior part of the inferior face there is a depression, which results from the convergence of the ambulacral areæ towards that point. The vent is in the posterior face. This species is from the _Neocomian_ strata of France; I introduce it to illustrate the characters of several other echinites, which the French geologists suppose to be confined to the so-called Neocomian formation; but which also occur in the Upper Greensand of Blackdown. Holaster is another genus of Spatangidæ established by M. Agassiz, for those echinites that are heart-shaped, with simple ambulacra converging towards the summit. The mouth is elongated transversely; the outlet is on the posterior face. A specimen first described in my _Foss. S. D._ (_pl._ xvii. _fig._ 9, 21), as Spatangus planus, is common in the Lower Chalk, and Chalk-marl, and abundant in the Firestone Malm-rock. * * * * * Our limits will not allow of a more extended notice of the fossil _Echinidæ_. The student should consult the Memoirs on the genera, now in course of publication at the Government School of Mines, by Professor Edward Forbes; the plates are exquisite, as works of art, and the descriptions all that can be desired. Mr. Dixon's work contains three excellent plates of cretaceous Echinites. Several chalk species are figured in my Foss. South Down. The numerous coloured figures of fossil sea-urchins in the Pictorial Atlas of Organic Remains, have already been mentioned. Geological Distribution of Echinites.--No vestiges of this order of radiata have been discovered in the Silurian deposits: the earliest known occurrence of any type is in the Carboniferous formation. The most ancient Echinidæ, according to the present state of our knowledge, are the Cidares, in the modified form previously noticed,--_Archæocidaridæ_ (_ante_, p. 322). In the Trias another type appears, _Hemicidaris_, which holds an intermediate place between the Cidarites properly so called and the Diadema. In the Oolite, and Jurassic formations, numerous forms are for the first time met with, constituting the genera Echinus, _Clypeus_, _Disaster_, _Holectypus_, Diadema, Nucleolites, &c. The Cretaceous seas swarmed with echini belonging to genera of which no traces have been found in earlier rocks; viz. Holaster, _Salenia_, _Micraster_, Salerites, Discoidea, _Ananchytes_, Cassidulus, &c.