DESCRIPTION OF CHALK CORALS. 245 
Turbinolia centralis, Roemer, Verstein. Norddeuts. Kreidegeb. p. 26, 1840 ; 
Geinitz, Charakt. Schichten u. Petref. siich.-bohm. Kreidegeb. p. 92, 1839-1842 ; 
and Gia von Sachsen, p. 131, 1843. 
The specimens submitted to examination presented considerable irregularities 
of growth (Tab. XVIII. figs. 3, 4, 9), consequent probably on having been attached 
to moveable bodies. One of those described by Mr. Parkinson adhered to ‘‘a 
plate of an Echinus” (op. cit. sup. p. 32). A tendency to an extension in one, 
possibly a vertical, direction prevailed evidently from the earliest period, as in 
the specimen delineated in fig. 3, a marked curvature existed close to the base, 
and two other equally decided cases occurred in the collection ; Dr. Mantell also 
gives a very prominent example of changed inclination. (Geol. S. Downs, t. 16. 
f.4.) This determination to a certain line of growth deserves, it is conceived, at- 
tention, as Cyathina Smithi will extend outwards from the perpendicular face of a 
rock, without any effort to attain a vertical position. The conical outline was chiefly 
limited to the earliest formed portions of the coral (figs. 1, 2, and lower part of 
other figures), slight variations taking place after the cylindrical contour was 
attained ; and Mr. Conybeare considers the elongated variety as ‘‘ perhaps only a 
different stage of growth”? (op. cit. p. 74) of the conical. One specimen (fig. 5) was 
cylindrical nearly throughout. The differences in diameter (figs. 4, 6) apparently 
depended more on relative vigour of development at an early period of existence, 
than on subsequently augmented powers (compare fig. 2 with figs. 3,4 & 6). The 
characters of the ribs also varied: near the base, when not overlaid by outer 
secretions, or not abraded, they were in very many specimens alternately large 
and small; but occasionally three of subordinate size occurred, the middlemost 
however being more prominent than the other two. As the coral extended up- 
wards, the ribs became almost uniformly broad, and in one case (fig. 4) nearly 
flat; but the greatest variations were adjacent to changes in direction of 
growth, distinct ridges being often obscure in those parts (figs. 3, 4); and if a 
succession of deviations occurred, the ribs were slender and more or less inter- 
rupted (fig. 3). The papillz likewise differed in distinctness and number, and 
they were occasionally not detectable even where the outer surface had not been 
overlaid. In some specimens the structure existed in the lower part, but was 
chiefly wanting in the upper; while in that represented by fig. 7, it was incon- 
spicuous throughout ; and in one case the exterior had a jagged appearance, 
due to the papilla being irregularly aggregated. Examples of much greater 
deviations in outer composition, arising, it was conceived, from injuries received 
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