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.
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.
SPONGITES IN CHALK AND FLINT.