I'llYLOGENY. 129 



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

 one of which inchules the Metopaaier species, the other the Mitntnter species. 

 Botli 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 tliau tlu-sc .1/. r<rrnututt, which 

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

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

 (see p. 55, PI. XTV, fig. 5), l)ut older specimens (see PI. XX fX, fig. I'l) accpiire 

 rugosities. 



The species of Mcfopaster, ]\f. (jiuuh-ofm^ and .1/. uncutiis, 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. quadratua has also acquired, as 

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

 The tyjie of ornament shows the sjiecies to be highly specialisi'd, and this fact, 

 tog-ether with its occurrence in the his-her 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 Mif raster show even more deciilcil evitlenee of \hc three phases 

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

 of the supero-marginalia. The infero-marginalia ai)peai- to pass directly from the 

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



Genera — Calliderma, NYMruASTER, and Pentaoonaster. 



This group tends to become smootli bolli in the Sunonian and in the Turonian- 

 Cenomanian. 



Turonian-Cenonianian forms include 6'. SinilJii.r, ('. Jul inn, .V. ('oomhii, N. 

 oIiyoj)hi,r, and N. iiumjhmhix, wliich are spiiious; N. riKjomK, which is rugose; N. 

 railiotitu, which is smooth on the oldei' proximal plates, but rugo.'^e on the younger 

 distal plates. 



Senonian forms include /'. 7"///'//"7(;/«/ and /'. hiinifns, wliich are spinous; P. 

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

 spines and are almost smooth. 



