94 BRITISH PALEOZOIC ASTEROZOA. 



also become more apical than marginal (see also the descriptions of species of 

 Pvomopalseaster in Schuchert's work cited above). It is rather difficult to make 

 out the exact arrangement of the intermediate plates lying between the primary 

 circlet and the primary interradialia. Schuchert states in reference to the 

 American species of Promopalseaster (p. 103) that these plates have no definite 

 arrangement, a conclusion with which I am disposed to agree in respect to this 

 English species. In the larger specimens these intermediate plates are irregularly 

 arranged, but in the smaller specimens (e. g. D. 75) the plates are more polygonal. 

 The base of one of the arms of D, 215 is figured (Text-fig. 52). The ossicles 

 appear at first sight to be scattered and displaced, but after careful investigation 

 and comparison with the account of the American species, I am convinced that the 

 irregular disposition of the plates is natural, and that in the genus there is a 

 tendency to lose that symmetry of the apical ossicles which is so conspicuous in 

 the more primitive genera. 



53 



54 



Text-fig. 53 (on left). — Outline drawing of ossicles on right-hand side of the distal portion of an arm of 



Promo/, alrfrjsler dizce (based on D. 10-t). x 6. 

 Textfig. 54 (on right). — Madreporite of Promopalrtasler eJi%(P. x 25 (abont) 



The central portion of the arm shows no serial arrangement of the plates. 

 Here and there larger plates, which are plainly radialia, may be seen. They are, 

 however, slightly displaced from the central line and separated by smaller ossicles, 

 which may represent the broken down intervening radialia. The remaining plates 

 between the bordering plates are undoubtedly adradialia, but these also are not in 

 definite series. The adradialia are present in considerable number and give 

 pronounced width to the arm. 



The supero-marginalia are larger than the remaining plates. When in position 

 they look of irregular shape, but isolated ossicles show the finger-form with but 

 little modification, l^he most proximal supero-marginal in the figure is the fourth 

 member of the series. The ossicle is isolated from its proximal neighbour in the 

 specimen, and in consequence its hollow articular face can plainly be seen. 



The arrangement of the ossicles of the apical surface is more regular as the 

 distal portion of the arms is approached, as seen in D. 104 (Text-fig. 53). This 

 shows radialia in series bordered by small triangular adradialia arranged in rows. 

 The appearance is strongly suggestive of that observed in P. wilsoni (Middle 

 Ordovician of America) and P. magnificus (Upper Ordovician of America), and may 



