314 DESCRIPTION OF CHALK CORALS. 
with an elongated development of the coral. Several opercula were observed, 
but they were less abundant than in axeal branches. In the thickest and appa- 
rently oldest examined fragment (fig. 6 g) the surface characters were obscure, 
and to a great extent obliterated by an overlying layer, secreted by the polype 
itself, and not adventitious. Figures 6 c, d, e, f, fully illustrate the form, range 
and variable position of the cells in thickened branches unequally worn down ; 
and the diversities might clearly have been as numerous as the sections which 
the specimen would have afforded. 
The intermediate structure changed in aspect with the condition of growth. 
In the direct centre of an axis it was small, but it gradually acquired importance 
as the visceral cavities radiated, constituting on the surface of unthickened frag- 
ments a prominent feature (fig. 6 d). The pores were very numerous and variable 
in form as well as, dimensions ; they were generally also open, but often par- 
tially or wholly closed, and the boundary slightly projected. Their inward range 
was not ascertained ; and their characters in thickened branches were generally 
very indistinct, either from mineralization or friction. ‘To the portion of the 
polype which occupied this intermediate structure, no doubt the successive ad- 
ditions were due ; and a comparison of the peculiarities exhibited by the latter, 
with those of other genera which have also outer increments, furnished or not 
with visceral cavities, will impress upon the mind the importance of a frequently 
neglected part of a coral. 
According to M. Milne-Edwards’s classification, Atagma on account of its 
operculum must be regarded as referable to the family Escharide’. 
Marginaria Roemeri. (Tab. XVIII. B. figs. 7, 7a to 7c.) 
Incrusting, cells egg- or pear-shaped, surrounded by a furrow; aperture large, 
semi-oval, arched margin thick, slightly raised, proximal margin nearly straight, 
very thin, depressed ; exterior covering to the cell nearly flat or very slightly 
convex, sloped downwards towards the aperture ; gemmuliferous capsules semi- 
globular, resting wholly on the next succeeding cell. 
The fossil (fig. 7) to which the above characters are applied, possesses appa- 
rently the structures assigned to Marginaria by Herr Roemer?; it even agrees 
with his species Marg. denticulata®, except that it wants the tubercle on the 
‘ Mémoire sur les Tubulipores, Ann. des Sciences Nat., 2nde série, Zool. tome viii., or Recherches 
sur les Polypes, p. 5. 
* Verst. Norddeutsch. Kreidegeb. p. 12. 5 Thid. p. 13. tab. 5. fig. 3 a,b. 
