HA RD WICKE'S SCIENCE- G OS SIP. 



135 



Nicholson in his "Manual of Palaeontology," re- 

 marks : "It seems jquite probable that the genera 

 Beloptaera and Belemnosis belong to the Belemni- 

 tidae." 



From these descriptions there appear to be many 

 fossil forms resembling — if not really allied to, or 

 developed from — the Belemnitidae. 



The development of knowledge in the departments 

 of Biology and Palaeontology, is fraught with the 

 ancient history of modern life, and is of great assis- 

 tance in spelling out the history of ancestral forms, or 

 the origin of a form of life under consideration. 



In the case of Belemnites, the ancestral form 

 from which they had probably developed, was 

 recognised by Buckland (" Bridgewater Treatise," 

 1837). He describes how a Belemnite is a modi- 

 fication of the shell of a Nautilus, and then proceeds 

 to show how the sheath or guard corresponds with 

 the apex of the straight cone of the shell, to which 

 there seems no equivalent in the apex of the coiled 

 up cone of Nautilus. The anterior horny cup 

 (alveolus) represents the anterior chamber of the 

 Belemnite, and contained the ink-bag, and other 

 viscera. 



We shall see in the course of this paper, that 

 although Buckland was right with regard] to the 

 Nautiloid ancestor of the Belemnite, he was wrong 

 in the comparison of the parts. 



Professor Lankester has studied the embryology of 

 some of our modern Cephalopodous molluscs, and 

 while agreeing with the probable development of the 

 Belemnite from a Nautilus-like ancestor, cannot 

 reconcile their modern analogues. 



The difficulty is with regard to the shell-gland in 

 the embryo of some of our modern Cephalopoda,* 

 and I quote at some length his remarks, f 



" The position and mode of development of the 

 shell-gland of the Cephalopoda exactly agree with 

 that of the shell-gland as seen in the other molluscan 

 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 development 

 of the shell-gland would have to be accepted. 



" But the consideration of the shell of the Belem- 

 nites 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 shell gland is the cavity in the mantle of the embryonic 

 mollusc in which the commencement of the shell is formed. 



+ " Quart. Journ. Micro. Science," 1874, quoted by Balfour. 

 "Treatise on Comparative Embryology," 1880. 



the facts ascertained as to the Belemnitidae. 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, Jout 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 Belemnite. 

 (See the figures in Brown's " Classen u. Ordnungen 

 des Thierreichs.") 



" In the Belemnites, the original nautiloid-shell is 

 small as compared with Spirulirostra. It appears to 

 be the largest in Huxley's genus Xiphoteuthis. 

 Hence in the series Spirula; Spirulirostra, Xipho- 

 teuthis, 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 pro- 

 portions 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 Belemnitidae 

 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 that point ; 

 it is more than probable that the living Dibran- 

 chiata are modified descendants of the Mesozoic 

 Belemnitidae. If this be so, the pens of Loligo and 

 Sepia must be traced to the more complete 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, 



