ECHINODERMATA—CYSTOIDEA. 459 



(Fig. 1774). The pores on opposite sides of the rhombs are united 

 by perfectly closed straight ducts which pass horizontally through 

 the plates, vertically across the line of suture, and produce a 

 transversely striated appearance (Caryocrinus, Fig. 1772). These 

 striate rhombs are thus generally visible only in weathered speci- 

 mens. They may be present on all plates or only on a few. In 

 Callocystites (Fig. 1779) and other related genera, the pore-rhombs 

 are reduced to comb-like rhombs, pectinate-rhombs, which are few 

 in number, lying on contiguous plates as do the pore-rhombs, but 

 the two parts of each are often separated externally by an interval; 

 often the two parts are of different form or size, or one of them 

 may become obsolescent (Fig. 1776). These structures had prob- 

 ably a respiratory function. 



Literature. 



1852-67. Hall, James. Pal. N. Y., vols. 2, 3. Ann. Rep. N. Y. 



St. Cab. Nat. Hist., 20 (Niagaran). 

 1858. Billings, E. Can. Org. Rem. decade 3 (Ordovicic cystoids 



of Canada). 

 1899. Jaekel. Stammesgeschichte der Pelmatozoen, I., Berlin. 

 1904. Schuchert, Charles. Smith. Misc. Coll. (Quar. Issue), vol. 



47, pt. 2 (Siluric and Devonic). 



Artificial Key to the Genera. 



A. Arms present I. 



I. Outer surface of plates deeply excavated XV. Comarocystites. 



I. Outer surface of plates not excavated i. 



1. Pores in rhombic figures, each one confined to two plates a. 



a. Pore-rhombs three ( calyx flattened ) VIII . Pleurocystites. 



a. Pore-rhombs many ii. 



II. Calyx subglobular VII. Caryocrinus. 



II. Calyx pear-shaped, with broad end above IV. Paleocystites. 



I. Pores in spherical triangles, each one divided among three plates. 



XVII I. Porocrinus. 



1. Pores simple or in pairs but not arranged in rhombs or triangles b. 



b. Sides of calyx with edges of plates only pierced by pores 22. 



22. Calyx with seven plates besides those of tegmen.. XIX. Zophocrinus. 

 22. Calyx with very numerous plates I. Eocystites. 



b. Sides of calyx with entire plate pierced by pores II. Holocystites. 



B. Arms absent II. 



II. Calyx attached by its entire under surface 2. 



2. Ambulacra straight XVII. Hemicystites. 



2. Ambulacra curved XVI. AgeUicriniis. 



II. Calyx attached by column 3. 



