30 
LETTER VII. 
OBSERVATIONS ON TURBINATED MADREPORITES CONTINUED 
PORPITAL OR SHIRT-BUTTON MADREPORITE SYNONYMA MA- 
DREPORES NOT FOUND IN A MINERALIZED STATE. 
Hardly any of the animalized parts of the creation are arranged with 
more difficulty than those remains of zoophytes which are obtained 
in a fossil state. The labours of the little creatures by which they 
have been modelled, frequently appear as if they had been capri- 
ciously and sportfully performed ; deviations from their ordinary 
model, apparently unnecessary, being frequently to be observed. The 
avoiding, perhaps, of some little obstacle to their labours, the repair- 
ing of some little injury, which accident may have occasioned to their 
operations ; or many other circumstances, not to be ascertained, may 
have occasioned these departures from the usual mode in which they 
had been wont to execute their assigned task. Hence the substances 
which they have formed may possess such anomalies of form, as may 
render it difficult to determine whether they should be considered as 
specific characteristics, or only as such accidental modifications of the 
structure of a species as give rise to varieties. The difficulty of making 
the necessary distinction, in these cases, must necessarily be much 
greater when, as is frequently the case with fossil specimens, we are 
deprived of the advantage of comparison, by having only one specimen 
on which to exercise our judgement. Especially since it also often 
happens, that its characteristic marks have been, in a great measure, 
effaced by the injuries to which it has been exposed, during a period 
of several thousand years. 
