iv BRITISH FOSSIL CORALS. 
In cases of this kind the distinction between the Polypi and the Bryozoa is always 
rendered easy by the most superficial examination of the soft parts of the animal; but it is 
sometimes a matter of great difficulty for the palzontologist, who is necessarily deprived of 
all such resources, and can only be guided by the peculiarities observable in the ossified 
tissues. 
In general, the distinction between Corals and Spongidee is also very easy, for the 
lamellar structure, so prevalent among the former, is never met with in the latter ; but in 
some Polypidoms (certain Milleporid for example), the vertical plates disappear, and the 
mural tissue becomes extremely porous, irregular, and abundant, so as to resemble much 
the reticulated mass formed by the stony skeleton of some Spongide, where the oscula 
and aquiferous canals are on the contrary more regular than usual. In cases of this kind 
it may he necessary to seek for distinctive characters in the internal structure of the 
Zoophyte ; and, independently of the benefit to be obtained by the microscopical investi- 
gation of the tissue itself, it will sometimes be found useful to examine the form of the 
tubular cavities which pervade the mass, and correspond either to the visceral chambers of 
the Polypi, or to the great aquiferous ducts of the Spongide ; for in the first instance 
they are always simple, whereas in the latter they are more or less ramified. 
§ IL. 
The external forms of Corals vary considerably, but are in general more dependent on 
the mode of aggregation of the different individuals produced by a common parent than 
on the mode of organization peculiar to the animals to which these tegumentary skeletons 
belong. Characters derived from these forms can therefore be but of little avail for the 
natural arrangement of Polypi; and the classification of these Zoophytes, like that of the 
higher animals, must be founded on the principal modifications observable in their struc- 
ture. It would lead us too far from the special object of this Monograph, if we were to 
enter on the investigation of the anatomical facts which alone can furnish satisfactory 
elements for such a classification ; but in order to facilitate the study of the Corals about 
to be described, it may be useful for us to revert to a few of the leading points in the 
structure of Polypi, and to define some of the expressions which we shall often have to 
employ.’ 
The ScrerencnyMa, or hardened tissue of Polypi, by which Corals are formed, is 
always a portion of the tegumentary system of these Zoophytes, but, as we have already 
stated, it may be produced in two very different ways. In some cases it is the result of 
a sort of ossification of the chorion or principal tunic of the Polypi; in others it grows on 
' For more ample details on this subject we must refer to our ‘‘ Memoir on the Structure and Develop- 
ment of Corals,” published in the Annales des Sciences Naturelles, 3™ série, t. ix. 
