DESCRIPTION OF CHALK CORALS. 253 
Diblasus, g. n. 
Fixed, invested externally by the mantle, outer surface irregularly thickened ; 
abdominal cavities, variable in characters ; lamellae numerous ; additional cavities 
produced on the margin of those previously formed, and in the mantle at irre- 
gular distances from it. 
Diblasus Grevensis, sp. n. (Tab. XVIII. figs. 14 to 28.) 
Amorphous, lobed, abdominal cavities immersed or projecting, conical or 
cylindrical ; lamellae unequal, near together, spinous on the sides, no distinct 
central structure, but a union of broadest lamellz ; surface partially ribbed, ex- 
ternal additions unequal. 
So far as the describer is aware, though this fossil is stated to be very abundant 
at Gravesend, no account of it has been published, nor is he aware of its having 
been figured. In the third volume of ‘ Guettard’s Memoirs',’ a Kentish coral is 
represented, and some resemblance may be detected between that delineation 
and the zoophyte under consideration ; but M. Guettard’s fossil is stated to be 
articulated ; ‘‘ En effet on distingue encore quelques-uns des nceuds qui forment 
les articulations du corail articulé”’ (Joc. cit.) ; and M. De Blainville has desig- 
nated it by the term Isis breviarticulata*. Had the cavities preserved fully their 
lamellz, a resemblance would possibly have existed with M. Goldfuss’s* Litho- 
dendron gibbosum, found in green, sandy marl of the chalk formation near 
Bochum, but the English fossil exhibits none of the characters of Schweigger’s 
genus*, equivalent to Oculina and Caryophyllia, in part, of Lamarck ; or of the 
peculiarities which distinguish Lithodendron, as limited by British palontolo- 
gists. M. Ehrenberg® conceives that Lithod. gibbosum may belong to his Sfe- 
phanocora, characterized partly by the cavities having a central area similar to 
that of Cyathina,—a structure however not detected in the best-preserved por- 
tions of the chalk fossil. 
The seven specimens submitted to examination, four belonging to Mr. Dixon’s 
cabinet, and three obligingly lent by Mr. Tennant, did not afford many data for 
detailed remarks. The mode of growth was best shown in that represented by 
‘ Mémoires sur différentes parties des Sciences et Arts, t. iii. pl. 58. f..5. p. 520. 
* Man. d’Actinologie, p. 503. * Petrefacten, p. 106. tab. 37. f. 9. 
* Beobachtungen, Syst., tab. 6. 
> Mr. J. Phillips’s Geology of Yorkshire, part 2. p. 202: also Sir R. I. Murchison’s Geology of 
Russia, vol. i. Appendix A. p. 597. © Beitrage, &c. p. 77. 
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