264 DESCRIPTION OF CHALK CORALS. 
trary the agreement in the nature of the abdominal cavities is greatest with those 
groups which have twelve or more lamella; and it is believed that the true 
position of the chalk fossil will be found to be intermediate between them and 
the ordinary corticiferous genera, and to supply a break in existing zoophytes. 
Spinopora Dixoni. (Tab. XVIII. figs. 38, 39.) 
Expanded, encrusting or branched; branches slender, tortuous, and irregu- 
larly divergent ; spinous tubercles small, unequally distributed ; wart-lke protu- 
berances variable in number and distinctness, with or without radiating ramifica- 
tions ; apertures to abdominal cavities poriform, minute, very numerous, often 
arranged in a circle around the spinous tubercles ; cavities crossed internally by 
transverse plates ; net-work forming the coral, microscopically porous. 
M. De Blainville! proposed the term Spinopora for two cretaceous corals to 
which M. De France had assigned in manuscript the name Pagrus. No descrip- 
tions or even figures of the two original species appear to have been published ; 
but M. De Blainville, who had examined the labelled specimens, added a third 
species by removing to the genus a Maestricht fossil previously named Ceriopora 
mitra by Prof. Goldfuss*. The essential characters ascribed to Spinopora are 
—‘‘un polypier calcaire, circonscrit, diversiforme, appliqué, adhérent par une 
face ordinairement concave et a cercles concentriques en dessous, réticulé et 
hérissé de tubercules épineux, entre lesquels sont des cellules poriformes en 
dessus.”’ (Op. cit. p. 415.) It was however in consequence of the external 
structures shown in the figures of Spin. mitra that the English chalk fossil was 
referred to the genus; and though they exhibit no traces of ramifications radia- 
ting from the wartose protuberances, yet as that structure was almost wanting in 
one of Mr. Dixon’s specimens (Tab. XVIII. figs. 39, 39 a), the presence or ab- 
sence of it is not regarded as an essential generic element. Among the corals 
described by M. Michelin® is one which resembles in some respects the English 
fossil, and is considered by him to be identical with the Chrysaora spinosa of 
Lamouroux*. In the observations on the coral it is not stated whether the de- 
termination rested upon an actual comparison with M. Lamouroux’s specimens ; 
but the figures in the ‘ Iconographie Zoophytologique ’ differ almost totally from 
" Man. d’Actinol. p. 415. 
* Petref. p. 39. tab. 30. f.13 a, 6; also, Man. d’Actinol. pl. 70. f. 3, not a good copy. 
* Icon. Zoophyt. p. 237. Chrysaora spinosa, pl. 55. f. 8. 
* Exposition Méthodique, p. 83. pl. 81. f. 6, 7. 
