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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, 



