"In the Belemnites the original nautiloid shell is small as compared with _Spirulirostra_. It appears to be largest in Huxley's genus _Xiphoteuthis_. Hence in the series _Spirula_, _Spirulirostra_, _Xiphoteuthis_, _Belemnites_, we have evidence of the enclosure of an external shell by growths from the mantle (as in Aplysia), of the addition to that shell of calcareous matter from the walls of its enclosing sack, and of the gradual change of the relative proportions of the original nucleus (the nautiloid phragmacone) and its superadded pro-ostracal and rostral elements tending to the disappearance of the nucleus (the original external shell). If this view be correct as to the nature of these shells, it is clear that the shell-gland and its plug has nothing to do with them. The shell-gland must have preceded the original nautiloid shell, and must be looked for in such a relation whenever the embryology of the pearly Nautilus can be studied. Now, everything points to the close agreement of the Belemnitidæ with the living Dibranchiata. The hooklets on the arms, the ink bag, the horny jaws, and general form of the body, leave no room for doubt on that point; it is more than probable that the living Dibranchiata are modified descendants of the mesozoic Belemnitidæ. If this be so, the pens of _Loligo_ and _Sepia_ must be traced to the more complex shell of the Belemnite. This is not difficult if we suppose the originally external shell the phragmacone, around which as a nucleus the guard and pro-ostracum were developed, to have finally disappeared. The enclosing folds of the mantle remain as a sack and perform their part, producing the chitino-calcareous pen of the living Dibranch, in which parts can be recognised as corresponding to the pro-ostracum, and probably also to the guard of the Belemnite. If this be the case, if the pen of _Sepia_ and _Loligo_ correspond to the entire Belemnite shell minus the phragmacone-nucleus, it is clear that the sack which develops so early in _Loligo_ and which appears to correspond to the shell-gland of the other Molluscs cannot be held to do so. The sack thus formed in _Loligo_ must be held to represent the sack formed by the primæval upgrowth of mantle folds over the young nautiloid shell of its Belemnitoid ancestors, and has accordingly no general significance for the whole Molluscan group, but is a special organ belonging only to the Dibranchiate stem, similar to--but not necessarily genetically connected with--the mantle fold in which the shell of the adult _Aplysia_ and its congeners is concealed. The pen, then, of Cephalopods would not represent the plug of the shell-gland. In regard to this view of the case, it may be remarked that I have found no trace in the embryonic history of the living Dibranchiata of a structure representing the phragmacone; and further, it is possible, though little importance can be attached to this suggestion, that the