Manc/iesier Memoirs, Vol. iv. (191 1), Auh II. 11 



tubercles will show the introduction of the radial element. 

 This stage might be called tubercidate concentric. {Fig. 3 B.) 

 The development of these tubercles is due to the 

 suppression of the concentric ornament along radial lines. 

 The suppression occurs along each radius and may affect 

 every concentric ridge, or alternate ridges only. In the 

 latter case alternate concentric ridges are suppressed along 

 alternate radial ridges only. In the Trigoniae the latter 

 rule holds, resulting in quincuncially arranged tubercles. 



After this stage development is possible along two 

 lines, either the concentric lines between the tubercles die 

 out leaving simple pustular ornament, ^purely tubercular 

 stage, or the concentric lines die out, and the specially 

 active spots become continuously active, giving radial 

 lines which show the last stages of the concentric 

 ornament in their pustulations, = ^?/(5^;r«/a/"^ radial stage. 

 These tubercles may die away leaving the purely radial 

 stage. {Fig. 3 C, D, E.) 



A still further stage of ornament is shown in the 

 Trigonise, where the alternating turbercles join forming 

 diagonals, these may be tiiberculate diagonal, or purely 

 diagonal, or even doubly diagonal ( = V). The develop- 

 ment of ornament then should be from concentric through 

 radial to diagonal. {Fig. 3 F, G, H.) 



The proof that this is so in the Trigonise, at least, 

 is seen in the Undulata^, where the double diagonal is 

 developed by rejoining of tubercles in ephebic stages of 

 individuals which, in early life, have pure concentric 

 markings. It is also seen in the ontogeny of the modern 

 forms, where the partial concentric umbonal markings are 

 replaced in later stages by tuberculate radial ornament. 

 Their phylogeny, as well as their ontogeny, also bears out 

 this order of evolution of ornament, since the Eocene types 



