252 CEPHALOPODA. 



There are certain difficulties in comparing the shell-gland of the Cephalo- 

 poda with that of other Mollusca which will best be rendered clear by the 

 following quotation from Lankester 1 : 



"The position and mode of development of the shell-gland of the Cepha- 

 lopoda exactly agree with that of the shell-gland as seen in the other Mol- 

 luscan embryos figured in this paper. We are therefore fairly entitled to 

 conclude from the embryological evidence that the pen-sack of Cephalopoda 

 is identical with the shell gland of other Mollusca. 



" But here forming an interesting example of the interaction of the 

 various sources of evidence in genealogical biology palaeontology crosses 

 the path of embryology. I think it is certain that if we possessed no fossil 

 remains of Cephalopoda the conclusion that the pen-sack is a special develop- 

 ment of the shell-gland would have to be accepted. 



" But the consideration of the nature of the shell of the Belemnites and 

 its relation to the pen of living Cuttle-fish brings a new light to bear on the 

 matter. Reserving anything like a decided opinion as to the question in 

 hand, I may briefly state the hypothesis suggested by the facts ascertained as 

 to the Belemnitidfe. The complete shell of a Belemnite is essentially a 

 straightened nautilus-shell (therefore an external shell inherited from a 

 nautilus-like ancestor), which, like the nautiloid 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 Belemnite 

 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 Spirnlirostra, a tertiary fossil, we have a shell very similar to that 

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

 Belemnite (see the figures in Bronn Classen u. Ordnnngen des Thierreichs). 



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

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

 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 



1 "Development of Pond Snail." Quart. J. of Micro. Science, 1874, pp. 

 37' 374- 



