6 4 



THE CYSTIDEA 



plates 20-24 in C. penniger is the first stage in the proliferation of those 

 plates to floor the food-grooves ; more advanced stages occur in other 

 species, thus leading to Glypton/sti^ Homocystis, Barrande (1877), Ordo- 

 vician and Silurian, Bohemia, resembles Cheirocrinus in the shape of the 

 theca, the number of pectinirhombs, and the position of the anus. The 

 material does not permit more precise comparison. Glyptocystis, Billings 

 (1854 and 1858), Ordovician, Canada. Theca ovoid (Fig. XXXIIL). Anal 



area less large than in Cheiro- 

 crinus, enclosed by 8, 13, and 14. 

 Plate 16 sunk on to 5, as in C. 

 penniger; thus 11 is pushed to 

 the right, and both 11 and 12 

 are reduced in size. Pectini- 

 rhombs 1 & 5, 1 & 6, 10 & 14, 

 10 & 15, 13 & 17, 15 & 16, 16 

 & 17, 17 & 18, 18 & 19; demi- 

 rhomb 10 & 16 ; imperfect 

 rhombs, 7 & 8, 11&12, 15 & 

 19. Main grooves five, passing 

 from over th e theca, by way of plates 

 15-19 ; all nearly reach the base 

 except anterior groove, which is checked by pectinirhomb 10 & 15. 

 The resemblance of the grooves to those of Callocystinae suggests that 

 that sub-family was derived from Glyptocystis itself; but this is nega- 

 tived by the different modification of the thecal plates. Pleurocydis, 

 Billings (1854 and 1858), Ordovician, N. America and Britain (Fig. 

 XXXIV.). Anal area so large as to occupy almost all one side of the theca 

 with its numerous small plates, the anus being at lower right-hand 

 corner of the area, which is bordered by plates 3, 2, 7, 12, 13, 14, 8. 

 Correlated with this is a flattening of the theca in 1. post, and r. 

 ant. plane, the atrophy of all main grooves except the two in that 

 plane, great diminution in size of plates 15-19, as well as 13. Pectini- 

 rhombs 1 & 5, 11 & 12, 10 & 14; the two latter are analogous 

 to, not homogenetic with, pectinirhombs 12 & 18, 14 & 15, in 

 Callocystinae. Each main groove ends in a single long and sturdy 

 brachiole, of the usual biserial structure, with stout covering -plates. 

 The stem is longer than usual in the family. The curious homo- 

 plastic resemblance to the Anomalocystidae (p. 52) led Haeckel to 

 place Pleurocystis in that family, though it differs in every essential 

 structure. Cystoblastus, Volborth (1867 and 1870), Ordovician, Russia 

 (Fig. XXXV.). Imagine a Cheirocrinus in which plates 10, 11, 12, 

 and 14 are still further pushed up between those of the fourth circlet 

 than they are in C. Volborthi, in which plate 13 has entirely dis- 

 appeared so as to compensate for the asymmetry induced by the 

 anus ; in which the arm-grooves, of the usual structure, are stretched well 

 into plates 15-19 and limited to those plates, as in C. sculptus: then you 

 will have such a form as Cystoblastus. It need only be added that the 

 anus is surrounded by plates 8, 14, and 19 ; and that pectinirhombs 

 1 & 5, and 1 & 6 remain as before, while demi-rhoiubs unite the plates 



