384 SPECIAL GEOLOGY. 



and are entirely marine, consisting of sponges, corals, eehino- 

 derms, shells, Crustacea, fishes, and " reptiles. Flints are 

 so constantly associated with organic substances that it is 

 scarcely possible to meet with one which does not contain 

 a sponge, urchin, or shell, or the impression of one or more 

 of these organisms. The choanite, which forms the nucleus 

 of many flints, and produces the beautiful markings 011 the 

 cut surfaces of polished pebbles, is a common zoophyte of 

 the white chalk. 



The Echinodermata were extremely abundant in the 

 cretaceous seas. The genera, Marsupites, Anancliytcs, Spa- 

 tanffus, JSchinus, Micraster, Ci/pJiosoma, Salenia, and Cidaris 

 greatly prevailed. Figures of the most common forms of 

 this sub-kingdom have been described and figured in our 

 Chapter on Paleontology. See figs. 157, 158, 161. 



SHELLS or THE CHALK. The accompanying figures 

 represent several of the most common cretaceous fossil 

 shells : 



FIG. 269. 1. Belemnites Listeri. 2. Terebratula carnea. 3. Lima spinosa. 

 4. Inoceramus Cuvieri. 



Of the above, the Inoceramus is of a thin, fibrous 

 structure ; the Lima spinosa is very characteristic of the 

 upper chalk ; the Terebratula is a well-known fossil, which, 

 originating in the earliest formations, and being profusely 

 distributed through the succeeding stages, has dwindled 

 down to a few species at the present day; the Belemnite 



