ii BRITISH FOSSIL CORALS. 



monly thus blended together. For us the word Coral, or Corallum, must be synonymous 

 with Polypidom, 1 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 beings of the same structure, and we 

 must circumscribe our researches within the boundaries of the group of Zoophytes, which, 

 in a Natural arrangement of the Animal Kingdom, is represented by the Class of Polypi. 2 



These Zoophytes are closely allied to Medusas, 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 Spongidse, which, till lately, have be.en 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 Spongidse 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 Spongidse ; and the Polypidom being the only part of these animals which is found 

 in the fossil state, it is sometimes hard for the palaeontologist 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 Volypier by the ■word Polypidom, which has of late been adopted 

 by some of the most eminent English zoophytologists, we deem it necessr/y 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 p;astic exudation moulded round the body 

 of the Polyp, and serving as a dwelling for these singular being.., nut 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," 

 Pohjpier, &c, migbt 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 Acalephse, as well as Polypi, etc. 



