FOSSIL MOLLUSCA. 379 
this order have powerful muscular arms, or tentacula, which 
surround the head, or upper part of the body ; some genera 
_ have no shell, but possess an internal skeleton, as the recent 
Sepiadze and the fossil Belemnitidee. Most of the testaceous 
Cephalopoda have a discoidal, univalve shell, which is divided 
internally by septa or partitions ; as the Nautilus. 
is, without any notch or groove ; in others it is notched or 
extended into a canal, or siphon, and this character has rela- 
tion to the respiratory organs: thus the Gasteropods, in 
_ which the water is jondacted to the interior by a muscular 
tube, or siphon, have the margin of the aperture of the shell 
channelled ; as in the Whelk, or Buccinum. Many of the 
land and fresh. water species have entire openings, and are, 
for the most part, herbivorous; while the greater number 
of the marine univalves eee the aperture indented or 
"notched, and are carnivorous.* Some of these mollusca, too, 
have a retractile proboscis, armed with minute teeth, by 
which they can rasp or bore into the shells of the species on 
_which they prey. There are some exceptions to the above 
rules, but the prevalence of the characters specified afford 
pretty certain indications of the fluviatile or marine nature 
of the originals. The application of these data to geological 
_ investigations will be considered hereafter. 
_ In the generic distinctions of the simple univalves, the 
_ form of the mouth is an important character ; while in the 
_ bivalves, the configuration of the hinge affords an equally 
convenient aid for their classification. 
| In many univalves the aperture or opening is entire, that 
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* The form of the aperture does not necessarily indicate fresh- 
water genera. Melanopsis, Pirena, and most of the Melanie have a 
channelled or notched aperture. Fresh-water univalves frequently 
_ have the spire corroded ; in a fossil state they can only be determined 
pit be fresh-water cea by their analogy to recent genera and sub- 
_ genera.—NVote by Mr. Woodward. 
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