Apt ychi from tlie Upper Chalk. 531 



an inch from the free margin there is a well-marked stain of darker 

 colour which follows the contour of the alveolar margin ; this is so 

 constantly present in well-preserved specimens that it is probably 

 due to, or marks a point of attachment of, the membrane which 

 united the pro-ostracum to the phragmocone. This darker marking 

 can also be observed along the outer and upper curved margin of 

 the aptychus. 



The roughened termination of the thin alveolar margin, and 

 corresponding irregularity of the proximal end of the aptychus, 

 have been already noticed ; from this it would appear there was 

 no fixed shelly attachment of the pro-ostracum to the phragmocone: 

 the union was one of membrane only, which would have allowed 

 a considerable amount of movement to this portion of the shell of 

 the Belemnite. This is also probably the reason why the aptychi 

 in the Chalk are not found attached to the guard ; the membranes 

 would speedily decompose after death, and in decaying become 

 separated from the heavier guard, which from its shape and weight 

 would naturally sink to the bottom of the ocean, whilst the mem- 

 branes containing the head, arms, and pro-ostracum would float away 

 and be deposited probably far away from the guard. 



One other point is worth}' of note, viz. what one might term the 

 suitability in point of size and shape. Fig. 4 represents the largest, 

 and Fig. 5 the smallest specimen found : both can be readily fitted 

 to appropriate guards from the same bed of Chalk ; whereas they 

 would be utterly out of proportion as opercula of the huge Ammonites 

 leptophyllus, which is also found in this locality, but in a much 

 lower zone. 



In the Marsupite zone of this neighbourhood Mr. Jebbett has, 

 however, met with a fragment of a very different and much thicker, 

 coarser aptychus, with numerous prominent points which project 

 nearly an eighth of an inch from the external convex surface. My 

 thanks are due to Mr. G. C. Crick for drawing my attention to 

 several nearly perfect specimens from Charlton and Bromley, in 

 Kent (Nos. 46,770, 48,741, 70,391), in the Natural History Museum, 

 Cromwell Boad, in every respect corresponding with a broken 

 specimen from Salisbury. These aptychi are very different from 

 those already described in this paper, and, from their size and the 

 fact of their occurring in the Marsupite zone, they may be fairly 

 assigned to Ammonites leptophyllus ; certainly they could not belong- 

 to any Belemnite. 



In the same zone of Chalk in which is found B. lanceolata and its 

 pro-ostracum, have been discovered specimens of rhyncholites, which 

 there is good reason for considering the beak of this Belemnite; 

 no Ammonite or Nautilus being known in this zone at this locality. 

 Two views of the upper mandible of a large Salisbury specimen 

 are given in Fl. XVI, Figs. 6, 7. 



Belemnitella qundrata differs in so many respects from B. lanceolata 

 and B. mucronata, both in the shape and proportion of the alveolar 

 cavity, as well as in the size and granular surface of the guard, 

 that one is not surprised to find how widely the small aptychi which 



