Manchester Memoirs, Vol. Iv. (191 1), No. 15. 9 



■ This bending or breaking is only seen in the n:iiddle 

 part of the valve, because the extreme deviation of the 

 radials is not felt by the upper diagonals, and the uplift 

 not by the lower. 



Fig. ya represents a specimen of T. irregularis with 

 its ornament interpreted according to the usual method, 

 and showing at + the introduction of an apparently 

 extra, intercalated row. 



Fig. jb represents the same specimen, with its tubercles 

 joined up on their right radials. Here no intercalated row 

 is visible, as the radials show that it belongs to the upper 

 part of row/! 



in Fig. 7^. 



The curious type of ornament is then due to the form, 

 and, if the form is merely ecological, T. irregularis must 

 lose its place as a species and become merely a variety of 

 T. clavellata. 



A similar reading may be taken for the ornament ot 

 T. scarburgensis, etc. 



T. dcedalea. This species is characterised by its 

 curious, flattened tall form, and the apparently double set 

 of ornament. 



