52 Dr. F. A. Bather — Studies in Edrioasteroidea. 



relative height is greater there, 

 millimetres are : — 



Height 



Greatest width . 

 Depth of portion exposed 



The measurements of such plates in 



a 



2-9 

 2-0 



•6 



1-8 



1-6 



'4 or less 



e 

 2-75 

 1-8 

 •5 



3 

 2-7 

 1-6 



•2 in column, 

 1-5 when alter- 

 nating. 



Thus the ratio of width to height varies from "88 to "59 in different 

 individuals, when plates of the same character are compared. 



The number of plates in a column is 21 in a with a total height 

 of 82 mm., 18 in c with a total height of 7 mm., about 16 in e where 

 the portion preserved has a height of 5*6 mm., about 23 in g, with 

 an approximate height of 8 mm. These numbers seem to imply that 

 the plates are closer than in S. sulcatum, according to the measure- 

 ment given by Aurivillius, who says 20 plates in 10 mm. But that 

 measurement does not seem to take into account the more crowded 

 plates at the two ends. His figure 11 suggests that there were in 

 that specimen 27 plates in a height of about 10 mm. Probably 

 there was no difference in this respect between individuals of the 

 two species with clearly marked columns. 



The amount of overlap is gauged by the proportion of depth of 

 a plate exposed, and this in vertical series is somewhere about one- 

 fifth. Aurivillius says of S. sulcatum " eine Schuppe nur die Halfte 

 oder oft nur -§- der nachststehenden freilasst", but later on he gives 

 the actual measurements for the middle region of the turret as *5 in 

 a length of 2*5 mm., i.e. one-fifth. 



As regards the mutual relations of the plates in a tier, Aurivillius 

 says : " Was die Anordnung in die Quere betrifft, stehen die 

 Schuppen jeder zweiten Reihe auf derselben Hohe." If we letter 

 the plates in a tier a to h, reading from right to left, then this 

 sentence conveys the impression that the arrangement is as follows — 



h 



f 



9 



It will be observed that a repetition of this order in successive tiers 

 would produce a line of the same letters ascending in spiral from 

 right to left, a overlapping b, b overlapping c, and so on — 



