EOACTINID^E. 203 



A good example of the artificiality of the use of the marginal plates as a feature 

 of primary importance in classification is seen in the case of the recent family 

 Linkiidse, which is placed by Sladen among the Cryptozonia, in spite of the fact 

 that the forms within the family belong to one of the great primary groups of the 

 Asteroidea, namely the Valvata, the majority of which are typically Phanerozonate. 

 Almost all the genera within the family have rounded arms, but when, as in Fromia, 

 the arms are angular, differentiated marginalia are present. 



-0. 

 -A*. 



H.P. 



Text-fig. 143. — Adoral view of a mouth-angle of Schuchertia wenlocki (from Royal Scottish Mus. no. -185). 

 Ad , first adambulacral ; M.P., mouth-angle plate ; 0., odontophor. x 10. 



It is possible that young forms may show differentiated marginalia and that 

 this differentiation may be lost in the older more mature specimens. In conse- 

 quence of this, Ludwig and others have stated as a general law that all 

 Cryptozonate forms have descended from Phanerozonate stock. Rather would the 

 explanation appear to be, that the so-called differentiated marginalia are the first- 

 formed plates, which obtain a start over the others and for some time remain 

 distinguishable. As the forms become more mature, the other rows of plates, 



M.F? 



Text-fig. 144. — Adoral view of a pair of mouth-angle plates (MP.) of Palasterina primxva 

 (from specimen in Mus. Pract. Geol.). x 10. 



provided that they have room to grow, catch up the early-formed plates, and the 

 distinction is lost. 



If the general argument be correct, we can use the changes in the shapes of 

 the marginal (or equivalent) plates to show that the general tendency of evolution 

 in the Schuchertia- lineage is to develop a flattish body, while in the Palasterina- 

 lineage the arms become progressively rounder. 



(d) The Genera ivithin the Family Eoactinidse. 



The general features of the Eoactinidas have been mentioned already (p. 178). 

 Their most obvious characters are the marked Asteroid shape, the broad adambu- 

 lacralia, and the simple flooring-plate character of the ambulacralia. Although the 



