PROMOPAL^ASTER. 91 



The form seems to be closely related to the Ordovician Mesopalaeaster, but until 

 more material is discovered it does not appear advisable to endeavour to give it an 

 exact specific diagnosis. It may be noted that the American Silurian members of 

 Mesoijalseaster show a parallel development of a considerable intermarginal area, 

 together Avith a thrust of the proximal supero-marginalia well towards the middle 

 of the disc area. 



Measurements. — r is 2*5 mm. 



Korizon and Localitij. — Silurian (Wenlockian) of Gutterford Burn, Pentland 

 Hills, Scotland. 



Sub-family Promopal^asterin^, Schuchert. 



1914. Schuchert, C, Fossilium Catalogus, Animalia, pt. 3, p. 6. 



1915. „ Bull. 88, U.S. Nat. Mus., p. 102. 



" Progressive Promopalfeasteridse with the interbrachial areas complex and 

 composed of the single axillary interbrachials [odontophors], more than one pair 

 of interbrachial marginals, and adambulacral plates. Podial openings in each 

 ambulacral furrow mainly in two columns, but proximally there may be two 

 additional ones in a length never more than one-third the ray." 



This sub-family contains one genus, Promoimlfeaster, Schuchert. The generic 

 name is given (n^ojuoc chief and acrrjp) because it includes the largest and chief 

 Starfishes of the American Ordovician. 



Genus PROMOPALJEASTER, Schuchert. 



Palseaster (part) of Authors. 



1914. Promopalxaster, Sclmchert, Fossilium Catalogue, Animalia, pt. 3, pp. 6 aud 24. 



1915. „ „ Bull. 88, U.S. Nat. Mus., p. 102. 



Generic Ghavaders. — See the definition of the Sub-family. 



A statement as to the evidence for the evolution of Promopalseaster from a 

 " Hudsonaster" through a " Mesopalgeaster" stage is given on p. 78. It is also 

 suggested there that Prornopalaiasfer could not have evolved from any known 

 Mesopalxaster. This is clear from the following argument. The earliest known 

 Fromopalseaster, P. wilsoni, is from the Middle Ordovician. In this form the 

 adradialia are " small, convex, overlapping V-shaped plates, which are arranged 

 with the point of the V directed towards the margin" (Schuchert, 85, p. 106). 

 P. marpiificas, the most advanced of the " Promopalgeasters," is also described by 

 Raymond as having the greater part of the apical surface " covered with smal 



