292 



CEPHALOPODA. 



par 



fossil forms may be expected from detailed palaeontological investigations and 

 perhaps also from more comprehensive ontogenetic researches. 



A comparison of the shell of Sepia with that of Belcmnites, such as was 

 attempted above, suggests a relation between the Dibranchia in general and 

 the Belemnitidae . This indeed seems a bold proceeding, since nothing certain 

 is known of the soft parts of these Cephalopods, but forms like Belemnoteuthis, 



a Cephalopod living in Triassic times with 

 well-preserved phragmocone and arms 

 carrying hooks, as well as Acanthoteuthis, a 

 Decapod much nearer the recent Decapods 

 and also armed with hooks (JAEKEL, No. 

 17), indicate with considerable certainty 

 that the Dibranchia or at least the Deca- 

 poda are to be derived from forms resemb- 

 ling the Belemnitidae. 



There can be no doubt that the shell of 

 the Belemnitidae was internal and was 

 almost completely enclosed in the mantle. 

 Of the transitionary forms, moreover, 

 Belemnoteuthis shows on the phragmocone 

 which is still provided with chambers and 

 siphon a large and distinctly bounded 

 proostracum (Fig. 139, po) recalling, in its. 

 shape, the Sepia shell. The rostrum, in 

 these forms, is either wanting or, when 

 present, is only a slight appendix to the 

 phragmocone. In this respect, these forms 

 would approximate to the recent forms 

 in which the shell has a posterior cup 

 (Ommastrephes, Oni/choteuthis, Taonius, 

 Leachia}. 



It is of interest that, in Ommastrephes, 

 regular transverse striation is found on the 

 hollow cone (Fig. 140) ; this is quite distinct 

 from the lines of growth in other parts of 

 the shell and may perhaps be regarded as 

 the last vestige of the chambering of the 

 phragmocone. [Gonatus Fabricii, according 

 to STEENSTRUP, has a series of chambers 

 at the end of its horny pen.] Such a view 

 does not appear unjustifiable, as Ommas- 

 trephes is among the most primitive of the extant Cephalopoda. JAEKEL 

 has already pointed out (No. 17) that Ommastrephes also, in the possession of 

 small hooks, shows;a primitive character and recalls the hook-bearing transi- 

 tionary forms mentioned above (Belemnoteuthis, Acanthoteuthis). 



FIG. 139. Shell of Belemnoteuthis 

 from the lower Lias, Lyme 

 Regis, somewhat diagrammatic 

 (original).* The shell is seen 

 from the tunnel-side with the 

 ink-sac lying (tb) upon it. In 

 the phragmocone (ph), the most 

 posterior part is wanting and is 

 indicated by dotted lines. The 

 partition-walls of the chambers 

 are seen on the surface owing to 

 the posterior portion being 

 broken away ; po, proostracum. 



* Fig. 139 represents a very instructive and as yet undescribed specimen 

 from the collection of Dr. O. JAEKEL, kindly placed at our disposal. Our 

 thanks are due to him also for revising the figure. 



