EOACTIS SIMPLEX. 207 



ambulacralia which are similar to those of Schucheriia (Text-fig. 124, p. 183), 

 except that the overlap of an ossicle on its predecessor is more pronounced. The 

 adambulacralia are broad, with an evenly swollen surface. One can just trace a 

 pustular ornament. The proximal adambulacralia are not differentiated either in 

 size or shape. They fit together so that the suture gives the appearance of a 

 twisted rope, exactly as is described as a generic character for the American form 

 Schoeuaster fimbriates, Meek and Worthen. I find that this character also occurs 

 in some species of Asjridosoma. The odontophor is large and prominent with a 

 swollen unornamented surface. It terminates proximally in a point which does 

 not quite reach the mouth-angle plates. A small ossicle intervenes here exactly as 

 is figured by Hudson (33, for Hudsonaster naiTawayi, reproduced in this Mono- 

 graph, PI. I, fig. 5). This small plate is not shown in the text-figure. 



M.P 



TLT. 



Am- 



Text-fig. 1-18. — Drawing of the mouth -region of Eoactis simplex. Ad., adambulacral ; Am., ambulacral ; 

 M.P., mouth-angle plate ; n >•., groove for nerve-ring, x 20. 



An infero-marginal fits on each side of the odontophor. Beyond this the 

 infero -marginal series disappears on to the apical surface. The extremity of the 

 arm pointing west in the photograph (PI. XV, fig. 8) shows one or two stout 

 plates in apical view which probably belong to this series. 



Dr. Bather {Joe. cit.) makes the following remarks about my original 

 description of the mouth-frame : " As an example of a very primitive mouth- 

 frame, Mr. Spencer (1914, p. 30) takes the fossil which he names Eoactis simplex. 

 His drawing (PI. I, fig. 4) shows a simple series of ambulacrals and adambulacrals. 

 At the proximal end of the groove the ambulacrals diverge, and the series is there 

 terminated on each side by a curved subtriangular plate, which Mr. Spencer 

 designates ' mouth-angle plate.' It is, however, clear from his drawing, no less 

 than from the specimen itself, that this plate continues the ambulacral series and 

 not the adambulacral, and this is further emphasised by the fact that the 

 depression for the first podium lies equally on this plate and on the adjacent 

 ambulacral. Further examination of other interradii in the fossil showed that this 

 ambulacral mouth-angle plate was actually overlaid by a paired adambulacral 

 element, though only the empty space that might have been occupied by such a 



