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BRITISH PALAEOZOIC ASTEROZOA. 



1 1' we examine modern genera we find that strongly differentiated marginalia 

 are only found in forms like Astropecten, which have an arm almost rectangular in 

 cross-section (Text-fig. 141). When a genus possesses a rounded arm, e. g. Nardoa 





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Text fig. 141. — Cross-section through the arm of a recent Astropecten. Ad., adanibulacral : 

 Am., ambulacral ; I.M., infero-marginal ; S.M., supero-marginal. 





(Text-fig. 142), the differentiation tends to become obscured. This may be explained 

 as follows : In the rounded arm there is no physical reason why any one ossicle 

 should be larger than another, provided that they all commence to be laid down at 



Ad 



Text-fig. 1 ti'.— Cross-section through an arm of a recent Nardoa. Ad., adanibulacral ; Am., ambulacral; 



R., radial. 



the same moment, for each ossicle has ample room to grow and the rows are not 

 subject to unequal lateral pressures. In an arm with angles at the side, especially 

 when the ossicles are crowded and consequently exercising pressure on one another, 

 the masses of calcite will tend to be pushed into the angles, where larger plates 

 will be built up. 



