220 BRITISH PALEOZOIC ASTEROZOA. 



a little produced, with three or five principal rows of tubercles above, combined 

 with a plated disc which fills up the angles; ambulacra rather shallow, of 

 subquadrate or slightly transverse ossicles, bordered by a single row of squarish 

 huge plates, the lowest of which (ad-oral adambulacral plates, Huxley ; angle 

 ossicula, Forbes) are large and triangular, bearing combs of spines (Upper 

 Silurian)." 



Unfortunately the specimen examined and described by Salter was not 

 /'. primseoa, but the form described later by Gregory as "P. bonneyi." Neither 

 Gregory nor later authors appear to have noticed this, and in consequence there 

 has been some confusion. 



I have placed in the genus several forms which some previous observers have 

 separated rather widely. These include " Lindatromaster " antiquus, Hisinger, 

 from the Wenlock (Middle Silurian) of Gotland, P. primseva, Forbes, and " P. 

 bonneyi" from the Ludlow* (Upper Silurian) of England, and " Pseud opalasterina " 

 follmanni, Stiirtz, from the Bundenbach (Lower Devonian) of German}*. 1 am 

 also able to describe a little new material from the Wenlock horizon of both 

 England and Scotland. All these forms possess one characteristic type of mouth- 

 angle plate (Text-fig. 144, p. 203), and agree in the rounded shape of the arm and 

 other general characters. 



The new form from the Wenlock of Scotland appeals to be a young specimen. 

 In many respects, particularly in the shape of its odontophor, it recalls Eoactis 

 (see p. 205). 



For the reasons stated below I think that we can limit the species as follows : 



(1) A Wenlock species, /'. antiqua, Hisinger, present also in the Ludlow 

 Beds and passing directly into — 



(2) The Devonian P. follmanni, Stiirtz. 



('$) A Ludlow species, /'. primseva, Forbes, which seems to be characteristic of 

 its own horizon and locality (passage beds between Lower and Upper Ludlow in 

 Lake District). 



The following table gives the diagnostic characters of these species : 



/'. primseva. — Form of medium size; R may be is mm. A comparatively 

 small number of ossicles in the interradii. Radialia and marginalia 

 can be identified. 

 /'. antiqua. — Form of medium size; R may be 17 nun. A large number of 



ossicles in the interradii. Radialia and marginalia can be identified. 



P. follmanni. — Large form ; R may be 113 mm. Arm much swollen with 



numerous similar rows of apical ossicles, which may be squeezed by 



post-mortem distortion into oral surface forming large interradii. 



The characters of the apical surface can be understood most clearly if 



we commence with a study of /'. primseca. In this form both radialia and 



marginalia are differentiated (Text-fig. 146, p. 204, and Text-fig. 157). The 



