EOACTIS. 205 



In addition to these two lineages there is a form, known from one specimen 

 only, which I have named Eoactis. It is not a Schuchertia, and differs from Palas- 

 terina in certain features which may prove to be only " age " features. At 

 present, however, I am disposed to give these features a somewhat greater value. 



A fourth genus is a multi-armed form known only from the Bundenbach 

 slates. It has been placed by Stiirtz in several genera and species. I am follow- 

 ing Schuchert in only recognising one genus and species, Vahvaxolaster t/reijori/i, 

 Stiirtz. On the whole the affinities of this form are with Palasterina rather than 

 Schuchertia, but we cannot say at present that it is merely a multi-armed 

 Palasterina. 



The genera of Eoactinidas may be defined as follows : 



Eoactis, Spencer. Mouth-angle plates rounded; arms straight; a very few 

 flat infero- marginalia in interradial angles. 



Schuchertia, Gregory. Mouth-angle plates narrow, high; arms straight and 

 flat; both infero-marginalia and adambulacralia recognisable on oral surface. 



Palasterina, McCoy. Mouth-angle plates swollen ; arms petaloid and rounded ; 

 adambulacralia bounding oral surface. 



Palseosolaster, Stiirtz. Multi-armed ; mouth-angle plates narrow and high ; 

 adambulacralia bounding oral surface ; arms rounded ; lateral and oral surface 

 of arm covered with small well-separated plates, each of which carries a long 

 spine. 



Genus EOACTIS, Spencer. 



1914. Eoactis, Spencer, Brit. Palaeoz. Asterozoa (Mon. Pal. Soc, vol. for 1913), p. 30. 



1915. „ Spencer = ? Urasterella, Schuchert, Bull. 88, U.S. Nat. Mus., pp. 173, 178, 186. 



1915. „ Bather, Geol. Mag. [6], vol. ii, p. 320. 



1916. „ Hudson, New York State Mus. Bull. 187, Twelfth Eeport of the Director, 1915, 



p. 135. 



Generic Characters. — See above. 



I commence my account of the members of this family by a description of the 

 form which I have named Eoactis simplex (see p. 30), It is known only from one 

 specimen and is from a comparatively high geological horizon. It may be the 

 young form of a species not yet recognised. Nevertheless it serves as a convenient 

 starting-point in the discussion of the origin and relationships of the other species. 



The form is almost cliagrammatically simple. The groove is floored with 

 ambulacralia of the flooring- plate type. Alongside these are stout adambulacralia. 

 The mouth-angle plates have no distinctive shape like those of Schuchertia 

 or Palasterina. There are a few other skeletal plates visible on the oral surface. 

 In the axils of the arms is a stout polygonal odontophor, and on each side of it a 

 definitely recognisable flat infero-marginal. The apical surface is unknown, but 



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