38 University of California Publications in Geology [Vol. 12 



tinguishing characters show that A. brewerianus has the wide open 

 petals, and more fundamentally a different type of ambulacral plates. 

 In Scutella the plates widen markedly from the ends of the petals 

 to the ambitus, whereas in Astrodapsis these plates start to broaden 

 about one-half the distance from the apical system to the margin, 

 which gives the abactinal portions of the ambulacra a more regular 

 appearance. Furthermore, the petals in Astrodapsis usually extend 



Dendraster 



Calaster 



I 



Scutella 



Astrodapsis 



Scutaster 



Fig. 2. Pliylogenetic tree for the more important Pacific Coast genera. 



nearer to the ambitus than they do in Scutella. That it has been 

 originally derived from Scutella is shown by its internal structure, 

 which is clearly of the scutellid type ; the flush petals also point to 

 the same derivation for it. Since A. brewerianus possesses all of these 

 astrodapsid characters it suggests that, although this is a rather prim- 

 itive form of this group, it is comparatively well advanced in its stage 

 of evolution beyond the original scutellid type. This can be accounted 

 for by the assumption that it had undergone an intervening develop- 

 ment between the scutellid type and the astrodapsid type in some 



