68 BRITISH PALEOZOIC ASTEROZOA. 



infero-marginalia may naturally be looked for in the distal, growing region of the 

 arm, where the newly formed ossicles may be expected to recapitulate the 

 ancestral arrangement. Here in very many forms the adambulacralia are equal 

 in number to, and alternate with, the infero-marginalia. Proximally the adambu- 

 lacralia tend to outnumber the infero-marginalia, possibly because (1) the infero- 

 marginalia tend to fuse and become larger plates, and (2) there is some actual 

 reduction in size, accompanied by multiplication in number, of the adambulacralia 

 themselves. In primitive genera belonging to other groups it will be seen that 

 the adambulacralia are equal in number to the infero-marginalia throughout the 

 arm. The proportionate length of arm through which the primitive arrangement 

 persists, appears to be some indication of the point at which lineages branched off 

 from the parent stock. 



(5) Schuchert (85, p. 48) endeavours to draw certain theoretical conclusions 

 from the apparent absence of ocular plates among the older Asteroidea, stating that 

 he " knows of but a single occurrence, in the Lower Carboniferous genus Neo- 

 palasaster.'" I do not agree with him on this point, for it seems to me that an 

 ocular plate can be seen on all the forms in which the tip of the arm has been 

 sufficiently Avell preserved for the plate to be recognised. 



Geographical Distribution. 



As Schondorf predicted would be the case when the forms were examined, there 

 is a great similarity between the American and British Paleeozoic Asterozoan 

 faunas. The relationship, however, is generic rather than specific, as may be seen 

 from the following descriptions. 



Family Hudsonasterid^, Schuchert, 1914. 



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



1915. „ „ Bull. 88, U.S. Nat. Mus., p. 53. 



" The most primitive known Phanerozonia. Small, five-rajed, heavily plated 

 asterids, with narrow am])ulacral furrows and slightly alternate ambulacralia. 

 The incipient interbrachial arcs are occupied by single axillary marginal plates. 

 Abactinally the rays have five columns of ossicles, the radials in the center being 

 bounded on either side by the supramarginals and inframarginals ; the latter 

 margin the rays, and the ossicles of adjoining columns alternate with one another. 

 There are no accessory ray-plates of any kind. The disc has a prominent central 

 di.sc-piece separated by a small number of accessory disc-plates from a ring of 

 five large basal radials and five large basal interradial ossicles. Spines rudi- 

 mentary and apparently restricted to the adambulacrals and inframarginals." 



I have quoted Schuchert's diagnosis in full, but I cannot agree with the 



