52 



BRITISH BELEMNITES. 



faces are distinctly marked with one stria from the apex, so that if these be regarded as 

 furrows, the apicial region is quadri-sulcate. The apices are in each case more pointed 

 and more recurved than is usual with B. paxillosus. A longitudinal section of the former 

 specimen shows the axis of the guard most excentric at the apex of the phragmocone, and 

 the youngest (included) forms to have been relatively much shorter than the older. 



In the accompanying sketches (Diagram 21) the smaller one shows the proportions of 

 the sheath in the youngest traceable form of this specimen ; the larger shows a middle-age 



DIAGRAM 2]. 



form (5) included in a third a full-grown specimen, whose probable extension is given. 

 In the three cases the proportion of the axis of the guard to the normal diameter (y, d)^ 

 taken at 100, is 140 for the youngest, 260 for the middle-age, 300 for the full-grown. 

 The proportion of the whole length of the guard to the contemporaneous length of the 

 axis is in the first case 260, in the second 180, and in the third (inferred, not measured) 

 it is nearly the same. In the first case the length of the whole guard is to its greatest 

 diameter about as 200 to 100, in the second about 350 to 100, and this proportion is not 

 materially altered with further growth. 



