AMORPHOZOA.
Fossil sponges take an important place among the organic remains of the former world, not only on account of their great variety of form and structure, but still more because of the extraordinary abundance of individuals in certain strata. In England they specially characterize the chalk formation,— extensive beds of silicified sponges occur in the upper green sand, and in some beds of the oolite and carboniferous lime stone. In Germany a member of the Oxford oolite is called the " spongitenkalk," from its numerous fossils of the present class.
Existing sponges are divided into horny, flinty, and limy, or " ceratose," " silicious," and " calcareous," according to the substance of their hard sustaining parts, which parts are commonly in the shape of fine needles, or spicula, of very varied forms, but in many species of sufficient constancy to characterize such species. The soft organic substance called " sarcode" appears to be structureless, and is diffluent ; it is uncontractile and impassive, but consists of an aggregate of more or less radiated corpuscles, in some of which the trace of a nucleus may be discerned. The larger orifices on the surface of a sponge are termed " oscula," and are those out of which the currents of water flow: these enter by more nume rous and minute "pores." The calcareous sponges abound in the oolitic and creta ceous strata, attaining their maximum of development in the chalk; they are now almost extinct, or are represented by other families with calcareous spicula. The horny sponges appear to be more abundant now than in the ancient seas, but their remains are only recognisable in those instances where they were charged with silicious spicula.
M. d'Orbigny enumerates 36 genera and 427 species of fossil sponges; and this is probably only a small proportion of the actual number in museums, as the difficulty of deter mining the limits of the species is very great, and many remain undescribed.
Pahrospongia and Acantiwspongia occur in the lower Silu rian ; and Stramatopora, with its concentrically laminated masses, attains a large size in the Wenlock limestone. Ste
ganodictyum, Spa rsisponyia, and species of &whin, are found 1. Siphoiiia pyriformis, Goldf. ; Grecnsand, Blackdown. z. Guettardia Tbiolati, D'Arch.; U. Chalk, Biarritz.
3. Vcntriculites radiatus, Mant.; U. Chalk, Sussex.
4. Manon osculiferum, Phil.; U Chalk, Yorkshire.
5. Fustilina cylindrica, Fisch. ; Carboniferous, Russia 6. Flabellina rugosa, D'Orb.; Chalk, Europe.
7. Lituola nautiloidea, Lam. ; Chalk, Europe.
8. Nummulites nummularia, Brug.; Eocene, Old World.
9. Orbitoides media, D'Arch.; U. Chalk, France. to. Ovulites margaritula, Lam. ; Chalk, Europe.
in the Devonian; and Bothroconis, Mamillopora, and Tragos, in the Permian or magnesian limestone. Several genera are common to the trial and oolites, and several more are peculiar to the latter strata. The Oxfordian sponges belong chiefly to the genera Eue Hippalimus, Cribrispongia, Stellispongia, and Curndisperagia. Their fibrous skeleton appears to have been entirely calcareous, and often very solid; their form is cup-shaped, or mammillated, or incrusting, and many have a sieve-like appearance, from the regular distribution of the excurrent orifices (oscula) over their surface.
The greensand of Faringdon in Berkshire is a stratum prolific in sponges, chiefly cup-shaped and calcareous, of the genera Scyphia and Chenendopora ; or mammillated, like Cnemidium and Verticillopora. The Kentish rag is full of sponges, which are most apparent on the water-worn sides of fissures. Some beds are so full of silicious spicula as to irritate the hands of the quarrymen working those beds. The green sand of Blackdown is famous for the number and perfect preservation of its pear-shaped Siphoniee (fig. 2, i) ; whilst those of Warminster are ornamented with three or more lobes. The latter locality is the richest in England for large cup shaped and branching sponges (Polypothecia), which are all silicified : the long stems of these sponges have been mistaken for bones. The sponges, chiefly &phonier, of the upper green sand of Farnham are infiltrated with phosphate of lime, and have been used in agriculture.