CONSTITUTION' OF CHALK AND FLINT. 
151 
Chalk and the Maestricht Chalk (which is rather higher in succes¬ 
sion) together, at pp. 227-229, and groups them thus :— 
Genera. Species. 
1. Monactinellidae (Monaxinellidae) . 3 6 
2. Tetractinellidse . 6 10 
3. Lithistidas . 30 70 
4. Hexactinellidee . 29 68 
5. Calcispongiae . 5 7 
Of Ho. 1 group, the parasitic boring Cliona is a well-known form, 
on account of the flint casts of the tubes and chambers, which it has 
made in Inoceramus shells, being often seen in the places where these 
shells, or fragments of them, have been imbedded in flint nodules. 
The shell has gone, leaving its shape and the impressions of its 
prismatic structure in the flint, whilst the chalk-mud once filling 
the little pipes and cavities has been exquisitely replaced by flint. 
Also in Belemnites and Echinoderm-plates Cliona has been noted, p 
In Ho. 2 group Dr. Hinde enumerates Ophiraphidites, Tethyopsis , 
Stelletta , Geodia ?, Thenea, and Pachystrella. 
Ho. 3 group comprises Dory derma, Phymatella, Callopegma , 
Trachysycon, Siphonia, Hallirhoa, Jerea, Thecosiphonia, Thamno- 
spongia , Pholidocladia, Ragadinia, Plinthosella, Phymaplectia , and 
others, from the Upper Chalk of South England. 
Ho. 4 group has many Upper-Chalk genera and species, for instance, 
Craticularia , Verrucocoelia , Guettardia , Coscinopora, Ventriculites , 
Cceloscyphia , Polylastidium , Cephalites, Placotrema , Plocoscyphia , 
Tremabolites , Ftheridgea , Toulminia , Camerospongia , Callodictyum , 
Diplodictyon, Cceloptychium , Stauractinella, Hyalostelia , etc. 
Ho. 5 group has for its English forms in the Upper Chalk 
Flasmostoma and Pharetrospongia. 
Foraminifera , etc. —It has been stated that one cubic inch of some 
Chalk contains hundreds of thousands of easily-visible small 
organisms, together with incalculable millions of minute granules, 
mostly recognisable as Coccoliths.* To show that the Chalk is 
rich with microzoa, I may mention that some years since an elderly 
friend of mine once went with his wife to see Windsor Castle, and 
they rested awhile in an arbour-like grotto made in the chalk of 
one of the terraces. Being a naturalist, he scraped off a little of the 
chalk; and at home, having washed and prepared the powder, he 
picked out from it all the Eoraminifera and Ostracoda which you see 
so neatly mounted on these two glass slides! 
If a piece of chalk be carefully examined either in slices 
prepared for the microscope, or as a carefully-disintegrated, washed, 
dried, and sifted powder, its numerous constituent particles are 
found to be generally some or other of the following, in different 
proportions in different specimens. 
* See Huxley’s “ Lecture on Chalk,” ‘ Macmillan’s Magazine,’ 1868, p. 396, etc. 
Very suggestive remarks on the Chalk and its history are given in Sir Charles 
Lyell’s “ Lecture on the White Chalk, with remarks on the theory of progressive 
development,” in ‘ Literary Gazette,’ January, 1851, Ho. 1824. See also Prof. 
C. Lapworth’s “ A Piece of Chalk,” in ‘ Science for All,’ vol. v. 
