il BRITISH FOSSIL CORALS. 
monly thus blended together. For us the word Coral, or Corallum, must be synonymous 
with Polypidom,' and signify the hard or ossified parts of the body of a Polyp. 
In treating of the “Fossil Corals of Great Britain,’ we must, therefore, exclude from 
our investigation the various organic remains which bear a certain resemblance to 
Polypidoms, but which do not in reality belong to bemgs of the same structure, and we 
must circumscribe our researches within the boundaries of the group of Zoophytes, which, 
ina Natural arrangement of the Animal Kingdom, is represented by the Cuass or Ponypt.? 
‘These Zoophytes are closely allied to Medusz, and in the actual state of science there is 
some uncertainty respecting the natural limits which separate these two groups; but the 
mode of organization common to both is so characteristic, that the most superficial 
anatomical investigation will always enable the zoologist to distinguish a Polyp or an 
Acaleph from the Bryozoa and the Spongidé, which, till lately, have been erroneously 
considered as belonging to the class of Corals. Polypi have a radiate structure ; a pro- 
tractile mouth, surrounded by non-ciliate tentacula; a large and well-organized digestive 
cavity ; but have no anus. In Spongidee no appearance of tentacula or of a stomach is 
ever met with; and in Bryozoa an intestinal canal, much resembling that of ordinary 
Mollusca, is always provided with two distinct openings, a mouth and an anus, the first of 
which is encircled by ciliated tentacula. The structure of the digestive organs is, therefore, 
characteristic in all these animals, and in most instances the radiate form of the tegumentary 
system will alone suffice to render the diagnosis of Polypi an easy task. But when the 
Polypidom is reduced to its most simple condition, it sometimes bears great resemblance 
to the calcareous or horny covering of certain Bryozoa, or to the reticulate skeleton of some 
of the Spongide ; and the Polypidom being the only part of these animals which is found 
in the fossil state, it is sometimes hard for the palzontologist to decide whether the 
organic remains that assume this form are in reality Corals, or whether they do not belong 
to one of the other above-mentioned Zoological divisions. 
Polypidoms may present two very distinct forms. Some, belonging to aggregate Polypi, 
are developed on the basal surface of these Zoophytes, and constitute a sort of stem in the 
1 In translating the French expression Polypier by the word Polypidom, which has of late been adopted 
by some of the most eminent English zoophytologists, we deem it necessary to guard the reader against the 
erroneous ideas which the etymology of that name might lead to. Till of late the nature of Corals was in 
general misunderstood ; they were supposed to be produced by a plastic exudation moulded round the body 
of the Polyp, and serving as a dwelling for these singular beings, but not forming a part of their organism. 
Such is far from being the case ; the corallum is a part of the animal, in the same way as the coating of the 
armadillo or the shell of the lobster belong to the structure of these beings. The words ‘‘ Polypidom,” 
Polypier, &e., might therefore be objected to, if their meaning was not generally known, and had not 
become independent of their etymology. 
2 The class of Polypi, reduced to its natural limits, corresponds to the Anthozoa of M. Ehrenberg, and 
to the sub-class of Radiated Zoophytes of Mr. Johnston. In the excellent work recently published by 
Mr. Dana, the same group is designated by the name of Zoophytes, which is usually employed in a much 
wider acceptation, and had long ago been given by Cuvier to the great division of radiate animals, com- 
prising Echinoderma and Acalephze, as well as Polypi, ete. 
