584 Profs. A. Dendy and J. W. Nicholson. Influence of 



€•324 x 93 = 301 mm. The displacement is in this case nearly 2 mm. 

 inwards (in accordance with the curvature ?). 



Fig. 11. — In this and the following cases there is a very definite displace- 

 ment of the median whorl from the centre of the spicule, a phenomenon 

 which has already been referred to in connection with fig. 5, and which will 

 be referred to again later on. 



The present specimen (fig. 11) is distinctly curved, and apparently very 

 near the critical stage, as indicated by the feeble development of the whorls. 

 A distinct primary central thickening is not recognisable, but the spicule 

 appears thickest at its geometrical centre, from which the median whorl is 

 displaced as much els 4 mm. towards the subsidiary whorl, which is at 

 22 - 5 mm. from the end. The whole length is 78*5, and tbe ratio becomes 

 - 287. Typical breadths are 1*4 and 2 - at distances of 22-5 and 35"0 mm. 

 respectively, giving ?i=4/5 on calculation. This corresponds to a theo- 

 retical ratio - 316, and a theoretical distance 24*8 mm. from the end to the 

 subsidiary whorl. This whorl therefore has a displacement of 2 - 3 mm. from 

 the theoretical position in the same sense as that of the median whorl. A 

 common factor of displacement, more effective for the median whorl, is 

 thereby suggested. 



Fig. 12. — This is a similar specimen to that shown in fig. 11, but shows the 

 primary central thickening clearly and at the geometrical centre. The total 

 length is 84 mm., and the distance from end to subsidiary whorl is 24*0, 

 giving a ratio 0'286. The whorls are relatively large, indicating a stage of 

 development slightly beyond the critical stage. 



There is again a displacement to the extent of 5 - 5 mm. of the median 

 whorl from the geometrical centre towards the subsidiary whorl. The value 

 of n is 4/5, and the theoretical distance is 26'5 against the observed value 

 23*5. A displacement of 3 mm. thus occurs in the case of the subsidiary 

 whorl in the same direction as the displacement of 5 "5 mm. in that of the 

 median one. The correspondence with fig. 11 is complete even quantitatively, 

 and a common agency effective in causing these displacements must be sought. 



Fig. 13. — This spicule is apparently very straight. Its total length is 

 75-0 mm., and the primary thickening is exactly central. The median whorl 

 is in this case displaced 2 - 5 mm. on the opposite side to that on which the 

 subsidiary whorl is situated, and we may therefore expect the latter to be 

 more displaced inwards than usual, in order to correspond. This phenomenon 

 actually occurs, for its distance from the end is 26 - mm., while the 

 theoretical value is 24 - 3 (n = 1, as the form is practically conical). The 

 extra displacement, as in figs. 11 and 12, is roughly half that of the median 

 whorl, although now the displacements are in the opposite direction. 



