Dec, 1920] 



Cystids and Blastoids 



41 



The angles of the quadrangular outline of the invaginated part are 

 strongly rounded, and terminate along the median line of the four basal 

 plates. Measured diagonally across the invaginated area, from one 

 angle to that diagonally opposite, the longer diameters of this area vary 

 usually from 14 to 16 mm. ; the depth of the invagination usually is only 

 2 or 3 mm., but may equal 4 mm. occasionally. 



Jaekel, in diagramming that specimen of Ca^locystis suhglobosiis for 

 which he proposed the name Sphcerocystites dolomilicus, also found one 

 plate missing, only one plate occurring in the area where usually the 

 two plates 9 and 10 should occur. Plates 11 and 5 appeared to have 

 enlarged, crowding the single plate representing both plates 9 and 10 

 toward the left. In consequence, the outlines of several plates have 

 been altered considerably. Plate 4 has an angular top, and plate 15 

 has a corresponding angular base. 



'Anus 



Fig. 2. The actual distribution of all pore-rhombs known with certainty in the 

 GlyptocystidcE, showing how a space is left clear where the gut may be supposed to 

 have pressed against the thecal wall. Copied from Bather, in Caradocian Cystidea 

 from Girvan, 1913, page 437. 



If the gut, pressing against the interior of the thecal wall, followed 

 a path across plates 13, near the sutures between plates 18 and 12, 17 

 and 6, 11 and 5, across plates 10 and 9, the lower part of 14 and the 

 middle of 8, to the anal aperture, then in many abnormal specimens 

 showing reductions in the size and ntmiber of plates or changes in 

 outline, these changes are more frequent along the supposed path of 

 the gut. (Bather, Echinoderma, 1900, p. 58, fig. 20). 



An entirely different type of abnormality is represented by the 

 second specimen of Ccelocystis subglobosus (Plate II, Figs. 5 A, B, D; 

 Schuchert, loc. cit., fig. 37) diagrammed by Schuchert. In this case the 

 upper part of the theca is enlarged by the introduction of four accessory 

 piates, nimibered 15', 16', 18', and 21', by Schuchert. The first three 

 of these accessory plates belong to the fourth primary circle of thecal 

 plates which includes plates 15, 16, 17, 18 and 19. The accessory plate 

 21' belongs to the fifth or circtim-oral circle. In each case the accessory 

 plate is assumed to have been added on the left side of the corresponding 

 primary plate, as though something had dragged the primary plates 



