PHYLOGENY. 129 



form, which was the ancestor of these two groups, early gave rise to two offshoots, 

 one of which includes the Metopaster species, the other the Mitraster species. 

 Both genera retained evidence of common ancestry by the possession of similar 

 ornament of the specialised " pustulate " type, and by the similar appearance of 

 their ultimate supero-marginalia, which, instead of being smaller than the rest of 

 the superior marginal series, are as large or larger than these. M. comutus, which 

 is the only species found in the lower zones of the chalk (Turonian), shows that 

 early specialisation set in. This form is spinous without rugosities when young 

 (see p. 55, PI. XIV, fig. 5), but older specimens (see PI. XXIX, fig. 12) acquire 

 rugosities. 



The species of Metopaster, M. quadratns and M. /meatus, have lost all spines 

 from the raised central area of their supero-marginalia, and, instead, possess 

 rugosities. The raised central areas of the infero-marginalia possess neither 

 spines nor rugosities, but are quite smooth. M. quadratus has also acquired, as 

 a frequent variation, a primitive type of ultimate supero-marginalia (see p. 98). 

 The type of ornament shows the species to be highly specialised, and this fact, 

 together with its occurrence in the higher zones of the Chalk, affords us an 

 explanation of the remarkable peculiarity of the terminal supero-marginalia on 

 the supposition that it is a catagenetic tendency. 



The species of Mitraster show even more decided evidence of the three phases 

 — spinous, rugose, smooth. These alterations only occur on the raised central area 

 of the supero-marginalia. The infero-marginalia appear to pass directly from the 

 spinous to the smooth stage without the intervention of a rugose stage. 



Genera — Callideema, Nymphaster, and Pentagonasteb. 



This group tends to become smooth both in the Senonian and in the Turonian- 

 Cenomanian. 



Turonian-Cenomanian forms include G. Smithise, 0. latum, N. Goombii, N. 

 oligoplax, and N. marginatus, which are spinous; iV. rugosus, which is rugose; N. 

 radiatits, which is smooth on the older proximal plates, but rugose on the younger 

 distal plates. 



Senonian forms include P. quinqueloba and P. lunatus, which are spinous; P. 

 obtusus, which very often possesses marginalia which have lost the majority of the 

 spines and are almost smooth. 



