222 THE MEDALS OF CREATION, Cuap. VII, 
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 paleeontology, 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.* 
ON THE SPONGES IN CHALK AND FLINTt.—From the dura- 
bility of the tissue of the flexible sponges, and the imperish- — 
able 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. Py 
Sponges so commonly form the nuclei of the nodular flints, — 
that some naturalists have ascribed the formation of the — 
layers and nodules of silex in the cretaceous rocks to these 
zoophytes : a supposition altogether groundless.t The various — 
* It has happened most unfortunately, that but recently Mr. Lons- 
dale, in the late Mr. Dixon’s beautiful work on Chalk and Tertiary 
Fossils, and Mr. Milne Edwards in his able Monograph in the Paleon- — 
tological Society’s Memoirs, have described many of our chalk Corals ¥ 
ped different specific and generic names. ‘¢ 
+ See Wonders of Geology, p. 300. This question is fully consi- i 
abeea in a Memoir entitled Notes of a Microscopical Examination of — 
the Chalk and Flint of the South-East of England, &c. by the Author, — 
in 1845, q 
Be 2s 
