SYSTEMATIC SECTIONS. 



61 



The late Dr. Vaughan (86) recently published the following diagram, which 

 most usefully illustrates lineage changes : 



Extinction. 



Phylogenetic old-age (exaggeration and 

 per-elaboration, structural stiffness 

 or want of adaptability). 



Adult stage (full size, all characters 

 matured). 



Growth according to definite laws of 

 structural variation. 















Culmination 

 ('burst') 

 and migration. ^ 





- 







- 























Young stage (small and simple, much 

 plasticity). 



Birth during a ' forcing ' period. 



The above diagram shows a sheaf of parallel gentes which arrive at the same structural 

 stages at the same time, but retain each its distinctive specific characters and time of burst. 

 Such a sheaf is more usual than a single line, and the laws of structural development can be 

 deduced from any one or more of the lines. 



It will be seen that the assumption that the evolution of Palaeozoic Asterozoa 

 follows these broad lines, is of great value when studying them. 



The Lineage Changes. 



Before dealing w^itli the details upon which the classification of the various 

 forms is based, it is best to give some account of the various lineage changes. 

 It will be seen that a knowledge of these changes is very important for a thorough 

 understanding of the classification suggested by Schuchert. 



No Asterozoa are known before the Middle Ordovician, and, as Schuchert 

 states ip]). cif., p. 31), "we may confidently add that the varied organisation of 

 those of the Middle Ordovicic proves that there is a long previous history of v/hich 

 palseontology knows nothing." 



The simplest Asteroidea known are the Hudsonasteridge, which have almost 

 exactly the structure met with in the early plated stages of the Recent Asteroidea. 

 The disc possesses a centrale surrounded by five or six accessory plates which are 

 ao-ain encircled by five primary radialia and five primary interradialia. The 

 marginalia are large and conspicuous, and there are prominent radialia but no 

 accessory plates, such as adradialia and ventrolateralia. A good idea of one of 

 these primitive forms can be obtained from an examination of Text-figs. 34, 35, 

 38, and 39 (pp. 71, 76). From the Hudsonasteridge certain lineages have developed 

 in ways which are, for the greater part, peculiar to these older Asteroidea. These 

 changes may be classified as follows : 



