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of slightingly, as a sort of jacket, an unimportant part, etc., that 
all conclusions arrived at by their study alone are considered as 
peculiarly liable to error. 
A shell, to begin with, ranks as a primary, essential part arising 
in an early stage of development from the shell gland common to 
the embryos of all forms of Mollusca. Subsequently, by its mode 
of growth it becomes a model of the external form, and at the 
same time a mould of the outlines of the internal soft parts to an 
extent which has not been fully appreciated. The shell is often, 
also, a permanent record of the series of changes which the form 
has undergone, from the time it first began to enclose the embryo 
until the death of the soft parts, since it retains the young shell and 
all the later stages of growth. Among Nautiloids and Ammonoids, 
it also contains the calcareous tube or so-called siphuncle, which 
exhibits remarkable and significant changes of structure and posi- 
tion following upon the development of the animal. This siphuncle 
connects the septa or horizontal partitions, which with their 
sutures vary with the age of the animal constituting a third record 
of changes and structural modifications. 
All these parts, the shell proper, the siphuncle, the septa and the 
sutures are in correlation with each other and together make an index 
to the life history of the individual, which is unequaled in some 
respects among other existing or extinct animals, 
A single shell, either from a living or fossil form, may present 
accurately the general history of the development of the young, 
the stages of the adult and old age. The results of heredity and of 
the action of endemic or traumatic diseases may also be detected, 
if one knows how to study and compare the remarkable and dis- 
tinct series of metamorphoses displayed by this external or protec- 
tive skeleton with those of congeneric forms. This can be done 
even when the young is not visible externally by breaking down or 
dissecting a well-preserved fossil and thus following the history of 
the shell backwards through all of its stages to the embryo. 
The researches of Beecher, Schuchert and Clarke among Brachio- 
poda have demonstrated that the shell and the internal brachial 
armature of these forms possesses similar life histories to those here 
described for the external and internal skeletons of the Cephalo- 
poda. Jackson has demonstrated similar phenomena among Pele- 
cypoda and Beecher among corals. 
The vertebrate skeleton has long been considered a standard, 
