THE ECHINODERMATA 269 



The micrasters of the White Chalk, of which M. cor- 

 anguinum is the latest, are famous from the work of Dr. 

 Rowe, who showed in 1899, from the study of large 

 numbers of specimens collected and arranged with care 

 according to their exact zones, that they went through 

 a gradual series of changes which did not bear a close 

 relation to the accepted distinctions of species. These 

 changes, in fact, run on parallel lines in separate stocks, 

 so that they may serve as zonal indices. The chief 

 changes may be summarized thus : 



(1) In general form they tend with the lapse of time 

 to become slightly broader and very distinctly higher, 

 while the ambital outline tends to change from wedge- 

 shape to ovate. 



(2) The carina appears and becomes more marked, the 

 highest point which was at first at the apical disc shifting 

 backwards ; finally a rostrum appears. 



(3) The anterior groove becomes deeper. 



(4) The mouth shifts forward and the labrum makes 

 its appearance; at first smooth at the tip, it becomes 

 more and more granulated; the labral plate changes 

 from triangular to oblong, and its tubercles from irregular 

 to linear in arrangement. 



(5) The amb-petals become longer and shallower. 



(6) The sub-anal fasciole, from being weak, becomes 

 very strong, and loses a slight tendency towards an 

 8-shape which it had at first. 



(7) The periplastronal area (the two posterior ambson 

 the under surface) from smooth passes to granulated and 

 finally to very coarsely granulated (mammillated). 



