92 BRITISH PALEOZOIC ASTKROZOA. 



convex triangular plates," although according to Schuchert there are " spaces 

 between the small triangular plates in this species" (p. 107). Very similar 

 adradialia are found on the younger more distal portions of the arm in the 

 British species, P. elizas (Text-fig. 53). So far as I know, no adradialia of 

 this type are found on any described species of Mesopalxaster, but Urasterella 

 primseva, which has undoubtedly descended from a primitive Asterozoan stock, 

 has similar plates. These facts are adverse to the conclusion of Schuchert 

 (p. 106) that "it seems probable, from the mature material studied, that the most 

 differentiated species of Proinopalxader, P. magnijicus, passed through ontogenetic 

 stages comparable to Riidsonaster, Mesopalgeaster granti, and Promopalseaster 

 bellulus.'^ The " Mesopalaeasters " and " Promopalasasters " appear to me rather 

 to be divergent lineages which follow a more or less parallel course of evolution 

 through certain of their earlier stages. 



A table of the American species is given on pp. 98, 99. Although many are 

 named, it seems as if the material really centres round a few forms. I could have 

 distinguished several species among the British material, but it seems to me that 

 the structure of the mouth-parts shows that really only one species is represented. 

 The British forms are only half the size of the largest American species, but they 

 are highly specialised. The want of real variety and the smaller size of the 

 British forms suggest that they are immigrants from some other centre. 



I have also to note that the specimen from which were drawn Text-figs. 18 and 

 20 of the Introductory Section to this Monograph, is a rather fragmentary example 

 not of a Uranaster but of a Promopalseaster. " Uranaster " elizx, therefore, is not 

 simpler in structure than Hudsonaster [Protopalsanster) iiarraioayi, as stated by me 

 on p. 21 of the Introductory Section, but is a highly modified form. There can be 

 no doubt, however, that the ambulacralia and adambulacralia of the younger 

 portions of the arm as figured (Text-fig. 54 a) are of a primitive type. 



The genoholotype of Proynoyalseaster is given by Schuchert as Palseaster 

 granulosus, Meek (not Hall, = P. speciosus, Meek). 



1. Promopalseaster elizae, n. sp. Plate IV, figs. 1-5; Text-figs. 51-58. 



1914. Uranaster elizse, Spencer, W. K., Introductory Section to this Monograph, i^p. 20, 23 ; Text- 

 figs. 18 and 20. 



Material. — All the specimens known are in the collection of Mrs. Gray. There 

 are fourteen imprints in all, and eleven of these possess counterparts. No specimen 

 is perfect, but altogether they present such a variety of aspect that the species can 

 be described in very considerable detail. The imprint D. 215, with its counterpart 

 D. 215c, is taken as the holotype of the species. Casts of these imprints are 

 figured, PI. IV, figs. 1-5. 



t 



