UKPOkT OF Till': DIRl'XTOR I9IO 257 



separate plates in the nepionic stage, for the plate numbered 3 in 

 the analysis shows a central trace of a suture and is clearly and 

 definitely notched exactly above it to admit the aborad angle of the 

 plate above 9 and 17 in figure 35. With these plates separate, the 

 arrangement of the circlet would be as in the Glyptocystidae where 

 the r.post.IR is always fused with the r.ant.IR of the same circlet. 

 These analyses all suggest an original circlet of five plates next to 

 the proximal stem joint, but the oral region shows neither trace of 

 an original pentamcrous symmetry nor of a still earlier triradial 

 extension of food grooves which would lead to a subsequent five- 

 rayed form through the forking of the right and left posterior 

 rays of the original three. It must be kept in mind, however, that 

 Sigmacystis is a specialized form. This is shown by the reduction 

 in the number of its plates, loss of primitive stem function and 

 change of position of mouth to side of theca. 



Sigmacystis does not possess a system of epithecal respiratory 

 canals and shows no trace of either pore-rhombs or pectinirhombs. 

 The large anal opening and inflated anal area suggest a more or 

 less elaborate system of anal respiration. Schizocystis Jaekel (1895) 

 and other members of the Glyptocystidae point the road to a total 

 loss of pectinirhombs and to plates with a raised central mound and 

 nodular surface, as in Sigmacystis. The affinities of this genus 

 will be discussed in a future paper. 



I have been under many obligations to the late Dr J. F. Whit- 

 eaves, to Dr John ]\I. Clarke and Dr Percy E. Raymond for gen- 

 erous loans of material and other help and to Dr Rudolf Ruede- 

 mann, Mr Edwin Kirk and Jacob \ an Deloo for assistance in pro- 

 curing literature. 



Note. Additional evidence for my conclusion that the b.vs. sur- 

 rounding a plate corner " are but external branches of one internal 

 tube or chamber" (see page 225, lines 10, 11) are to be found in 

 a recent and very valuable paper by Mr Frank Springer " On a 

 Trenton Echinoderm Fauna at Kirkfield, Ontario " .[Canada De- 

 partment of IMines, Memoir No. 15-P] which w^as received while 

 my present paper was in press. I refer to p. 43, lines 12 to 

 16 inclusive and plate V, figures loe, lib and iic. These figures 

 show definitely the branching of the inner portion of a corner b.v. 

 not below the theca but within the walls of the suture itself. These 



