222 



BRITISH PALEOZOIC ASTEROZOA. 



a central spine stouter than the remainder. This ornament must have been very 

 similar to that in Palseosolaster (p. 239). The arm in P. follmanni shows no 

 differentiation of marginalia. It was clearly more rounded and flexible (PI. XVI). 

 The position of the arms in PI. XVI, figs. 6, 7, proves that they had the 

 power of bending upwards over the disc. Fig. 6 shows only the base of the 

 arms in oral view. The other side of the specimen has been cleared of the matrix 

 and shows the remaining portions of the arms folded over the disc. The upward 

 flexure is very clear in the specimen photographed in fig. 7. 



The fact that the arms are preserved in this condition, and the general good 

 state of preservation of the specimens, suggest to me that the form lived buried in 

 the mud with the tips of its arms flexed and searching for food at the surface of 



-Ad. 



\r. 



M. 



Text-fig. 159.— Wash-drawing of an angle of the disc of Palasterina primxva (from Sedo-wiek Mus ) 

 Ad., adambulacral ; Ir., interradial plate; M., madreporite ; M.P., mouth-angle plate, x 15. 



the bottom. In this respect it contrasts with Schuchertia, which appears to have 

 lived near the surface (p. 210). 



The constitution of the oral surface is most typically shown in P. primaeva (Text- 

 fig. 161, p. 225). The greater part of the surface is formed by the prominent 

 adambulacralia. The oral interradii in the various species, and even in individual 

 specimens, vary considerably in extent. Those of /'. primaeva usually contain but 

 a few plates. In both /'. follmanni and /'. antiqiia these areas may be quite exten- 

 sive (see, e.g., Text-fig. 165, p. 229, and Text-fig. 168, p. 232). The photographs 

 on PI. XVI give the explanation of these discrepancies. When the arms are high 

 and the body swollen, post-mortem compression may cause the extensive lateral 

 surfaces in the interradii to be forced oralwards and so make the form look as 

 if it had a large flat disc. 



