MOLLUSC A. 209 



nautilus-like ancestor), which, like the nautiloicl shell of Spirula, has 

 become enclosed by growths of the mantle, and unlike the shell of Spirula, 

 has received large additions of calcareous matter from those enclosing 

 over-growths. On the lower surface of the enclosed nautilus-shell of the 

 Beleninite the phragmacone a series of layers of calcareous matter have 

 been thrown down forming the guard ; above, the shell has been continued 

 into the extensive chamber formed by the folds of the mantle, so as to 

 form the flattened pen-like pro-ostracum of Huxley. 



" Whether in the Belemnites the folds of the mantle which thus covered 

 in and added to the original chambered shell, were completely closed so as 

 to form a sack or remained partially open with contiguous flaps must be 

 doubtful. 



" In Spirula, we have an originally external shell enclosed but not added 

 to by the enclosing mantle- sack. 



" In Spirulirostra, a tertiary fossil, we have a shell very similar to that of 

 Spirula, with a small guard of laminated structure developed as in the 

 Beleninite (see the figures in Bronn Classen u. Ordnungen cles Thierreiclis). 



" In the Belemnites the original nautiloid shell is small as compared 

 with Spirulirostra. It appears to be largest in Huxley's genus Xiplwteuthis. 

 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 Belemnitidte with the living 

 Dibranchiata. The booklets on the arms, the ink-bag, the horny jaws, and 

 general form of the body, leave no room for doubt on 1 that point ; it is 

 more than probable that the living Dibranchiata are modified descendants 

 of the mesozoic Beleninitidte. If this be so, the pens of Loligo and 

 Sepia must be traced to the more complex shell of the Beleninite. 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 chitiiio-calcareous pen of the 

 living Dibranch, in which parts can be recognised as corresponding to the 

 pro-ostracuni, and probably also to the guard of the Beleninite. If this 

 be the case, if the pen of Sepia and Loligo correspond to the entire Beleni- 

 nite shell minus the phragniacone-nucleus, it is clear that the sack which 

 develops so early in Loligo and which appears to correspond to the shell- 

 glaiid 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 primaeval up- 

 growth of mantle-folds over the young nautiloid shell of its Belemnitoid 

 ancestors, and has accordingly no general significance for the whole 

 Molluscaii group, but is a special organ belonging only to the Dibranchiate 

 stern, similar to but not necessarily genetically connected with the 



B. E. 14 



