﻿SCHUCHERT] SILTKIC AND DKVOXIC CYSTIDEA 21/ 



Fig. 24. — Anah sis of Hallicystis imago Hall, based on no. 35,060, 

 U. S. N. M., and on one of Walker's specimens. 



HALLICYSTIS ELONGATA Jaekel 



Hallicystis elongata Jaekel, Stammesgeschichte der Pelmatozoen, Berlin, 

 1, 1899, p. 288. 



The description, translated, is as follows: 



" Theca greatly arched, 35 to 40 mm. high. Anns in the middle 

 of the length, with a depression about its vicinity. Theca above the 

 anus still convexly arched. Upper pectinirhomb on plates 14 and 

 15 of medium size, with 14 dichopores. Plates flat, rippled, folded 

 toward the middle, and with concentric lines along the margins. 

 Upper Silurian (Niagara limestone), Chicago." 



Holotype in the University of Strassburg, Germany. 



Tctracystis n. gen. 



Definition. — Apiocystinse with the theca elongate snbquadrate, 4- 

 sided in transverse section, and composed of 20 plates. These are 

 arranged as follows : 



Basal row has plates 4, 1, 2, 3. 



Second row has plates 5, 6, 7, 8, 9. 



Third row has plates 10, 11, 12, 14. 



Fourth row has plates 16, 17, 18, 13, 19, 15. 



Fifth row has deltoids 23 and 24. The former has a large hydro- 

 pore and a small madreporite. 



Anal area prominent, bounded by the thecal plates 7, 8, 13, and 14. 

 and composed of an outer circle of many small pieces and the acutely 

 pointed pyramid of 6 pieces. One basal and 2 large upper adjoining 

 pectinirhombs, with numerous dichopores. 



Ambulacra 4, simple, slender, and extending to the column. These 

 are R I, R II, R IV, and R V. Brachioles very slender, widely 

 separated, and therefore comparatively few in number. 



