60 BRITISH PALEOZOIC ASTEROZOA. 



SYSTEMATIC SECTIONS. 



Section A. — Family Hudsonasterid^, Scliuchert ; 



„ Peomopal^asterid^, Schuchert; 



„ Xenasterid^, Schondorf ; 



„ Uranasterid^, no v. 

 The forms included in the above families are those Palaeozoic Asterozoa which 

 are most similar to the Asteroidea of the present day. They possess — 



(1) Distinct rows of supero- and infero-marginalia. All Recent Asteroidea 

 possess this double row of marginalia either persistent through life or in the early 

 ontogenetic stages. 



(2) An apical madreporite. All Recent Asteroidea possess an apical madre- 

 porite. 



(3) Adambulacralia for the greater part confined to the oral (actinal) surface 

 and only assisting to form the margin at the extremity of the arms. This is a 

 well-known Asteroid character. 



(4) An " adambulacral " mouth-region. Recent forms may possess an " ambu- 

 lacral " mouth-region, but embryological research shows that the early stages 

 have a mouth-region of the primitive " adambulacral " type. 



Further, Schuchert has shown that the early members of these families possess 

 the same primitive arrangement of the plates of the disc as do the early stages in 

 the development of the Recent Asteroidea. It is seen, therefore, that the evidence 

 of relationship of these families with the Recent Asteroidea is very complete. 



Method op Study. 



The method of study which I have adopted is that based on the work of 

 Hyatt and other paleeontologists and followed by myself when investigating the 

 Asteroidea of the Cretaceous period. It is briefly this : 



(1) That the world has been peopled by successive races (or lineages), each 

 of which has gone through a period of rise (elaboration) and fall (regression). 



(2) That the early stages of a lineage may be recognised by the fact that the 

 forms are small, comparatively rare and unspecialised. The more mature stages 

 are characterised by large, more specialised forms, which occur in comparative 

 abundance. In the old age of the lineage there are marked loss of ornament and 

 other specialised characters, and the forms are obviously tending to disappear. 



(3) That in the various lineages there is frequently an assumption of parallel 

 characters which may make individuals of different descents look, at first sight, 

 very similar. Care must be taken consequently in classification to select characters 

 peculiar to the lineage and not characters paralleled in other lineages. Neglect of 

 this point has caused several classifications of Palaeozoic Asterozoa to be fallacious. 



