372 E. RAY LANKESTER. 



think it is certain that if we possessed no fossil remains of 

 Cephalopoda the conclusion that the pen-sac is a special 



Diagram of vertical rigiit-aud-lel't seel ion tl\rou?h mautle-region of an 

 embryo Loligo. ep. Epiblast. y. Food-yelk. m. Mesoblast. m' . 

 Deep-layer of cells (query, hypoblast) separating embryo from food- 

 yelk, shs. Open pen-sac. 



development 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 Belemnitidse. 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 Spirilla, has become en- 

 closed 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 phragma- 

 cone — 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 sac or remained par- 

 tially open with contiguous flaps must be doubtful. In 

 Spirula we have an originally external shell enclosed but not 

 added to by the enclosing mantle-sac. 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 Belemnite (see the figures in Bronn, ' Classen u. Ordnungen 

 des Thierreichs'). In the Belemnites the original nautiloid 



