Tethea: having a tough outer skin; siliceous spicula in bundles, and radiating from the compact nucleus to the periphery. Halichondria: (from _silex_ and _cartilago_) siliceous spicula in a cartilaginous mass. Grantia: calcareous spicula in a gelatinous mass. [Sidenote: FOSSIL SPONGES.] M. D'Orbigny seems to believe that with the exception of the horny _Cliona_, all the fossil sponges had originally calcareous skeletons,--"qu'ils n'ont jamais été cornés, mais que leur tissu a toujours été calcaire et pierreuse;"[196] which is certainly not the case, for abundant examples of fossil keratose sponges occur. [196] Cours Elémentaire de Paléontologie, torn. ii. p. 208. I have selected a few genera for the illustration of the subject, and shall describe them under the names that are most familiar to the British scientific collector: doubtless sooner or later some competent naturalist will undertake the elucidation of this department of palæontology, and construct a classification and nomenclature based on natural characters; till then the student will find it hopeless to attempt to learn the ever-varying names of genera and species applied to fossil Porifera and Polypifera, by different observers.[197] [197] It has happened most unfortunately, that but recently Mr. Lonsdale, in the late Mr. Dixon's beautiful work on Chalk and Tertiary Fossils, and Mr. Milne Edwards in his able Monograph in the Palæontological Society's Memoirs, have described many of our chalk Corals under different specific and generic names. [Sidenote: SPONGITES IN CHALK AND FLINT.] On the Sponges in Chalk and Flint.--From the durability of the tissue of the flexible sponges, and the imperishable nature of those which have a siliceous or calcareous endo-skeleton or framework, their fossil remains generally occur in a fine state of preservation, and in immense quantities, in the sediments that were deposited in those parts of the ancient sea-bottoms, originally inhabited by these zoophytes. Even the relics of the keratose species, the _Halichondria_, whose structure consists of siliceous spines imbedded in a cartilaginous mass, are equally abundant. Sponge-spicula are everywhere met with in the chalk, flint, and greensand, and many layers in the cretaceous strata are almost entirely composed of them.