32 



PEOF. H. H. SAYI>]SE1?T0X AISD ME. A. E. TErEMA]S' [vol. Ixxiii, 



The septum was 14^ mm. broad, and measured 9*25 mm. from 

 dorsum to venter. The datum-plane taken passed throug-h the 

 centre of the ammonite at right angles to the plane of coiling, 

 and touched the sejDtum tangentially in its median line. 



The general form assumed by septa has been aptlv described by 

 S. P. Woodward^ as 



' nearly flat in the middle and folded round the edge like a shirt-frill, where 

 they abut against the outer shell-Avall.' 



The antei-ior folds are referred 



Fig. 4. — Contoured plan of the 

 adult septum of Dactvlio- 

 ceras commmie (see JPl. III. 



fia- !)■ 



[The shaded portion is the flattened 

 area ; the thick lines represent 

 the posterior folds, or lobes : 

 the thin lines represent the 

 anterior folds, or saddles. The 

 contours are di-a-n-n at interrals 

 of 0-25 mm.] 



althouo^h it should 



to as saddles and the posterior 

 as lobes. The extreme antero- 

 posterior range from crest of sad- 

 dle to trough of lobe was 6*5 mm. 



The graph shown in fig. 3 (p. 31) 

 was made by finding the relative 

 areas of septum in front of succes- 

 sive contour-lines. It shows that 

 nearly 52 per cent, of the area of 

 the septum lies between contours 

 only 0*75 mm. apart. This 

 portion of the septum, with its 

 extensions into the folded region, 

 is shown ^Aith dotted shading in 

 fig. 4. This flattened area is con- 

 centrated chiefl}' in the centre of 

 the septum, and coincides approx- 

 imately with the datum-plane. 

 The central area is crossed by 

 very slight undulations, which 

 miite the corresponding folds of 

 the opposite sides, or of the venter 

 and dorsum, in all three genera. 

 In higiily compressed or depressed 

 ammonites these undulations be- 

 come more or less strongly de- 

 veloped folds. 



From the graph it is also 

 apparent that the area occupied 

 by the posterior folds is nmcli 

 greater than that occupied by the 

 anterior folds, and further that the 

 former are more acute than the 

 latter. It thus appears that the 

 septum as a whole is convex for- 

 wards, as stated by J. F. Blake 2 ; 

 be noted that sections across the septum in 



^ ' Manual of Palseontology ' 1872. p. 286. 

 2 ' The Yorkshire Lias '1876, p. 263. 



