288 



CEPHALOPODA. 



developed Cephalopodan shells. Among the living Dibranchia, the shell 

 which appears to approach most nearly to those of the extinct forms is that 

 of Spirilla ; here we find a spirally coiled and chambered shell which, how- 

 ever, is grown over and enclosed by the mantle. This approximates most 

 nearly to the shell of certain Belemnites which was also an internal shell 

 though, so far as its chambered portion (phragmocone) was concerned, it was 

 less highly developed than that of Spirula. 



The shell of the Belemnite is not, like that of Nun til its or most Ammonites, 

 spirally coiled, but is straight like that of the Orthoceratidae (Nautiloidea). 

 It is characteristic of the Belemnite that in addition to the part of it, the 

 phragmocone, that may be regarded as corresponding to the actual cham- 

 bered Cephalopodan shell (that of the Ammonites 

 and Nautilus) there is a calcareous pointed external 

 investing piece, the so-called guard or rostrum. A 

 good example of this latter is afforded in the fossil 

 Spirulirostra (Fig. 135) the phragmocone of which 

 is curved and thus bears a certain resemblance to 

 that of Spirilla, but has, in addition, dorsally and 

 late rally, a large conical rostrum. It has also been 

 maintained that traces of such a rostrum are to be 

 found in Spirula, although these were not found by 

 us in an examination of a large number of shells.* 



The shell of the Belemnite is, as already men- 

 tioned, straight. Two parts can be distinguished in 

 it. The chambered part {phragmocone) is provided 

 with a siphon and is surrounded by the actual shell- 

 wall (the ostracum) ; this latter becomes specially 

 dilated towards the head of the animal as the so- 

 called proostracum, and is not here followed by the 

 phragmocone. This first portion of the shell might 

 .be compared with the Ammonite or Nautilus shell, 

 but on the side opposite to the proostracum, sur- 

 rounding the posterior part of the phragmocone and 

 prolouged in the same direction, there is a second 

 part, a very large rostrum, which is usually the 

 only part of the whole Belemnite to be preserved 

 in a fossil condition. That this shell was internal, 

 i.e., surrounded by the mantle, seems to be highly 

 probable from observations made on forms resembling Belemnites but better 



-r 



Fig. 135. —Longitudinal 



section through the 

 shell of a Spiruli- 

 rostra (restoration). 

 /J/, phragmocone ; r, 

 rostrum ; si, siphon. 



* The position of Spirula as a branch of the Belemnite stock connected to 

 it by forms like Spirulirostra seems to us doubtful. While the position of 

 the siphon and the orientation of the shell with regard to the body would 

 seem to favour this view, the constitution of the shell, on the other hand. 

 with its well-retained chambers and the siphon renders it improbable that 

 it has undergone a process of degeneration which led to the total loss of the 

 rostrum. The shells of other recent Cephalopods, in which such degeneration 

 took place, underwent, in consequence, great change of structure. We must, 

 in any case, take into account the view that Spirula may have separated from 

 bhe Decapodan stock before acquiring a rostrum. 



