THE CYSTIDEA 



63 



circlet is reduced in size and sometimes quite atrophied. Pectinirhombs 

 on 1 & 5, 12 & 18, 14 & 15, as usual. Main grooves five, pass nearly to 

 base of theca ; one or more of the grooves may bifurcate once towards 

 the distal end. Sjrfiaerocystis, Hall (1859), Silurian, Maryland. Theca 

 spheroidal. Main grooves four, each with three to six branches. Plates 

 undetermined ; pectinirhombs, anus, and hydropore situated as in Lepado- 

 crinus. Strobilocystis, White (1876), Devonian, Iowa, resembles Sphaero- 

 cystis, but the branches have become "small secondary arm- grooves 

 extending obliquely downward from each side of the principal grooves." 

 SUB-FAMILY 3. GLYPTOCYSTINAE. Besides the negative characters of 

 irregularity in the shape of the plates and in the distribution of pectini- 

 rhombs, the only feature common to the included genera is the evidence 

 they offer of descent from a simple form like Cheirocrinus. The number 

 of pectinirhombs connects Gheirocrinus with Cystoblastus and Glyptocystis ; 

 the enlargement of the anal area connects it with Glyptocystis&nA.Pleurocystis; 

 the relation of the third circlet of plates to the fourth is another link with 

 Cystoblastus. Genera Cheirocrinus, Eichwald (1856 and 1859 ; see also 



Fio. XXXII. 

 Analysis of Cheirocrinus penniger, modified from Fr. Schmidt. 



Schmidt, 1874, under Glyptocystis), Ordovician, Russia, and North America. 

 The chief departures from the archetypal arrangement of plates are 

 correlated with an increase in size of the anal area, which is covered with 

 numerous small plates, and surrounded by plates 7, 8, 12, 13, and 14. 

 This in C. penniger, the type-species, induces the vertical fission of 18 

 (or intercalation of 18), and sinking of 12 on to '2; the pushing of 15 

 to the right over 10, and sinking of 10 between 9 and 5 on to 4 ; the 

 pushing of 17 to the left over 11; the consequent sinking of 16 between 

 10 and 11 on to 5. In G. Volborthi the plates of the third row are not 

 sunk, but raised between those of the fourth row ; this is a change in 

 the direction of Cystoblastus. The distribution of the numerous pectini- 

 rhombs varies with the species, and, to some extent, with the individual ; 

 those on 1 & 5, 1 & 6, with the demi-rhombs 1 & 4, 1 & 2, are constant. 

 The arrangement in one specimen of G. penniger is shown in Fig. XXXII. 

 Reversions to the older and simpler type of rhomb structure occa- 

 sionally occur (e.g. 13 & I8a in the figure). The five main grooves 

 pass between plates 20-24 to plates 15, 16, 17, 18a, and 19 ; they are 

 rather wide and fringed with short brachioles. The threefold division of 



