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an assemblage of defective objects—beautiful they may be, but 
they are biologically incomplete without this member. Seeing 
that the shell of a Gastropod is its skeleton, and the operculum 
part of that skeleton, it is strange that in collections even of 
recent shells this integral part of the organism is so often 
absent. Unskilled and unscientific collectors are to blame; yet 
it is only fair to admit that such a deficiency is becoming less 
common, for a spirit of clearer scientific insight is animating 
those who study the Mollusca as the chief invertebrates of the 
Animal Kingdom, and the result is apparent in the fact that 
foreign specimens of Mollusca are at present generally accom- 
panied, at least in museums, with this appendage, whilst 
examples of British species, from the facility of procuring 
specimens, are very rarely “put up” without the part. Of 
course the frequency of the occurrence of complete specimens 
is generally in proportion to the abundance of the species. But 
a different set of conditions altogether embarrasses the palzon- 
tologist in his researches, which is evident when we consider 
how the fossil Mollusca became entombed in their parent rock. 
The dissolution of the softer parts, such as the ligament in the 
Conchifera and in the Gastropoda, of the animal itself to which 
the operculum belonged, convinces us that the chance of 
preservation of such a delicate structure as an operculum must 
have been rare, and even when this was shelly, instead of horny, 
the chance of survival was inconsiderable, so that if the part 
became petrified at all, it must have owed its preservation 
to a shelly or calcareous nature. Accordingly, we must not 
veil the fact that we absolutely know next to nothing of the 
opercula of Gastropods in the fossilized state, so that cases 
which accident occasionally discloses are quite exceptional. 
One or two of these rare instances it may be interesting to 
mention; and first, we would touch on the odd and singularly 
shaped operculum of that Lower Silurian genus Maclurea 
(Lesvrur, 1818) allied to Bellerophon, but whose affinities 
are not clearly known, once thought to be confined to the 
Chazy Limestones of the United States of America, until it 
was detected in 1854, by Mr. Cartes Pracu, in the Highlands 
