CARADOCIAN CYSTIDEA FROM GIEVAN. 



431 



plate 10 (Jaekel's I' 2>) lies immediately under plate 15, so that the rhombiferous 

 suture is almost horizontal. This feature is one of those that connect Glyptocystis 

 with Cheirocrmus (text-fig. 40) ; it is not found in Mimocystis, but is a character- 

 istically Cheirocrinid modification of the regular archetypal plan. In this respect 



Text-fig. 39. — Analysis of Lepadocystis Moorei, based on Meek's figures (1871 and 1873). 

 Radials notched by subvective grooves. Pore on plate 23. 



the primitive Callocystid Lepadocystis P. H. Carp. ( = Meekocystis Jaekel, text-fig. 

 39) and most of the Callocystinae more nearly resemble the archetype. In them it 

 is not this suture with its rhomb that bars the way, but the enlargement of rhomb 

 14-15 ; and if Glyptocystis had retained their more primitive arrangement of plates, 

 the anterior groove could easily have passed between the rhombs of plate 15. The 





Text-fig. 40. — Analysis of C/i«i>omnMs ^e«w?'g'cr, modified from Fr. Schmidt, 

 as in Bather (1900). (See further § 293. ) 



shifting of the suture seen in Glyptocystis and Cheirocrinus is due to the enlargement 

 of the periproct, which has shoved the neighbouring upper plates out of their archetypal 

 position. In the Callocystinae the periproct is relatively small, and the shifting of 

 the plates correspondingly slight. Now which view is the more reasonable : that the 

 process of evolution was reversed in these respects ? or that Lepadocystis and its allies 

 descended from the archetype independently, through some link at present undis- 

 TRANS. ROY. SOC. EDIN., VOL. XLIX. PART II. (NO. 6). 57 



