H96 



THALLOPHYTES. 



occur abundantly in the White Chalk, and Giimbel has shown that they 

 are present in many limestones, even in those of the older Palaeozoic 

 formations. Two distinct forms of coccoliths are known, which have 

 been described by Huxley under the names of " discoliths " and " cyatho- 

 liths." The former of these (fig. 1362, a) are minute calcareous discs, 

 averaging 40*00 to 50*00 ^ ncn m diameter, provided with a raised rim on 

 one side Q? and c), and having a central corpuscle or nucleus. The 



Fig. 1362. — Coccoliths and coccosphere. a, A " discolith " seen in front view ; b and c, " Dis- 

 coliths " viewed edgeways; d, "Cyatholith" seen in front view, showing the outer transparent 

 zone, the inner granular zone, and the central corpuscle ; e, " Cyatholith" seen in profile ; f, A 

 "coccosphere." All the figures are greatly magnified. (After W. B. Carpenter and Wyville 

 Thomson.) 



" cyatholiths " vary from t ^-qq to ff qVo i ncn m diameter, and when viewed 

 sideways (fig. 1362, e) are seen to have the form of a shirt-stud, and to 

 consist of two concavo-convex calcareous discs, of which one is smaller 

 than the other and is united to the concavity of the larger by a short 

 stalk. When viewed in front (d), the cyatholith presents itself as an oval 

 or circular body, composed of two concentric zones surrounding an oval 

 central corpuscle, the inner of the two zones being more or less distinctly 

 granular, while the outer one is transparent. Cyatholiths occur in vast 

 numbers in the mud of the deep sea in their detached condition, but they 

 are also found aggregated into spherical masses, which may reach T qo 

 inch in diameter, and are known as " coccospheres " (fig. 1362, /). The 

 precise nature of the coccoliths and coccospheres is at present uncertain ; 

 but it has been suggested by Carter that the " coccospheres " are con- 

 nected with the reproduction of species of Melobesia. 



The only remaining genus of the Corallinece which demands con- 

 sideration is Lithothamnion* under which are included the well- 



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Fig. 1363. — Lithothamnion ramosissimum, a "Nullipore," from the " Leitha-Kalk" of the 

 Vienna basin, a, Portion of a mass, of the natural size ; b and c, Transverse and vertical sec- 

 tions of the same magnified 320 diameters. (After Giimbel.) 



known and widely distributed calcareous Alga which are commonly 

 spoken of as " Nullipores," and which are exclusively marine in 



