THE PAST AND PRESENT OF THE CUTTLEFISHES. 113 



or horny " pen " shell of the common squid. But the halting-places 

 on the way diminish the apparent length of the journey, as they lessen 

 the seeming irregularity of the path. The simple rudimentary shells of 

 our two-gilled cuttlefishes, are to be regarded as the degenerate 

 remains of structures fully developed in their ancestors. To this idea, 

 their succession in time bears faithful witness ; and to its correctness 

 the connecting links, accessible to us, plainly testify. 



Starting with the perfect four-gilled shells as ancestors of the 

 imperfect pens and shells of living cuttlefishes, we may find in some 

 such straight shell as that of Orthoceras a type of shell persisting 

 onwards to the Trias from the Silurian as likely a form as any other 

 to have evolved the newer races in part. We discover next in the ex- 

 tinct Belemnites of the Mesozoic rocks a first halting- place. Already 

 we are in the domain of the two-gilled cuttles ; for the belemnite was 

 a kind of squid or calamary, and possessed the ten arms of its race. 

 Evolved from some orthoceras-like or straightened nautiloid ances- 

 tor, the belemnite shell bears proof of its descent in its structure. 

 Here we discover a chambered portion resembling the straight nau- 

 tilus shell ; and, combined with the chambers, is an anterior part, 

 the pro-ostracum, and a posterior portion, the guard. Next in order 

 in the list of connecting forms comes the little Spirilla, in which an 

 originally outside shell becomes enclosed by the mantle. The shell 

 of Spirula (fig. n), often termed the " post-horn" shell from its shape, 

 exhibits a spiral form ; the coils lying in one plane, but not being 

 in contact as are the whorls of the nautilus shell. The shell of 

 Spirilla is practically a belemnite shell, minus &guard and pro-ostracum. 

 In Spirula, therefore, the transition from an external nautilus-like 

 shell to an internal shell is clearly to be witnessed. In Spirulirostra, 

 a fcssil and extinct form found in the Tertiary rocks, we have a shell 

 like that of Spirula, but possessing in addition a guard imitating that 

 of the belemnites. Within the actual domain of the zoologist, are the 

 shells we discover in the existing sepias. In these animals a limy 

 plate represents the result of degenerative action consequent upon 

 altered life and habits which no longer tend towards shell-perfection. 

 The chambers of the originally outside shell of the spirula type, and 

 the chambered part of the inside shell of the belemnite type, have 

 together disappeared in the limy plate or " cuttle-bone " of sepia and 

 its allies a rudiment of its chambered portion being indeed still 

 recognisable in its so-called " mucro," and we have thus left to us in 

 the sepia, merely the rudiments of the belemnite's " chambers " and 

 "guard." The horny "pen" of the squid represents a still more 

 modified structure on which the laws of development have operated, 

 modifying and deleting the shell-rudiment until it remains merely as 

 an interesting landmark in the evolution of its possessors. 



Thus the history of the cuttlefish shell forms an important chapter 



