DESCRIPTION OF CHALK CORALS. 311 
nevertheless it is not a Heteropora. No allusion is made in either the ‘ Petrefacta’ 
or in the ‘Manuel d’Actinologie’ to an intermediate structure, an exceedingly 
important part, whether simple or otherwise, when the viscera and oral append- 
ages are regarded as only portions of the polype. The coral under consideration 
has, however, a complicated intercellular composition, and on it depended the 
peculiarities of the protuberances with the unconformable position of the cells 
successively accumulated. An intermediate animal structure necessarily existed 
in Ceriopore ; but if the encrusting series of visceral cavities be regular and 
uniform ', the portions of the polype which produced the general framework, and 
prepared the receptacles for the digestive organs, must have been differently 
constituted from those of Mr. Dixon’s coral. Again, no allusion is made to the 
apertures differing in outline or size from the cell, or to their being covered 
under any condition by a solid, calcareous plate, the whole upper surface having 
possibly been membranaceous and perishable ; but in the English fossil, though 
the cell near its upper termination is round and ample, yet the aperture is 
variable in shape, of limited dimensions, and often provided with a solid cover- 
ing in both young (fig. 3) and thickened branches. The cells likewise pre- 
sent many marked differences. In Ceriopore they apparently maintain a con- 
stancy of character; while in the cretaceous production, they have in the centre 
of a stem an attenuated, downward extension of considerable range, conforming 
in this respect to the upward growth and circumscribed area of the axis ; 
whereas in the added protuberances, the cells are relatively short, and variously 
disposed, often radiating horizontally but irregularly from a centre ; the succes- 
sive increments also being totally unconformable. This aggregate of differences 
is considered sufficient to warrant a separation from Ceriopora ; and as the observed 
structures are not known to co-exist in any established genus, it is proposed to 
distinguish the chalk zoophyte by the term Atagma (a deprivative, tayna ordo), 
in allusion to the want of agreement in the position of the cells, constituting 
successive layers. 
Atagma, n. g. 
Cellular; cells variable in shape, grouped in cylindrical branches and irregularly 
accumulated in successive layers; mouth operculated ; intermediate structure 
porous. . 
Atagma papularium. (Tab. XVIII. B. figs. 6 to 6 g.) 
Branched ; branches dichotomous, the first formed or axeal portion round and 
" Consult Goldfuss, Cer. micropora, pl.10. figs. 46, 4c. 
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