290 CYSTOIDEA, BLASTOIDEA, AND HOLOTHUROIDEA. 



with its corresponding pore on the other half by means of 

 canals or grooves, which may or may not be visible exter- 

 nally. The pores in the pectinated rhombs appear to be the 

 external openings of canals which perforate the test verti- 

 cally, and open into a series of flat tubes or parallel redupli- 

 cations of a thin shelly membrane ; and this structure leads 

 to the belief that the sea- water was admitted to their in- 

 terior, and that they thus were enabled to serve in the 



Fig. 173. A, Pectinated rhomb of Glyptocystites multiporus (Billings). B, Pectinated 

 rhomb of Pleurocystites (Billings), c, l\vo plates of Callocystites Jewetti (Hall), showing the 

 pectinated rhombs ( p). All enlarged. 



aeration of the perivisceral fluid. While many Cystideans pos- 

 sess regular pectinated rhombs, of the structure above noted, 

 others appear to have a completely imperforate test (Crypto- 

 crinus, Malocystites, Trochocystites} ; while in other cases there 

 are pores or fissures distributed without any very definite 

 arrangement over most or all the plates (Echinosphcerites, 

 Caryocystites, Palceocystites, &c.) 



As regards the distribution of the Cystideans in time, they 

 are not only wholly extinct, but they are exclusively con- 

 fined to the earlier portion of the Palaeozoic period. They 

 appear to have commenced their existence in the Upper 

 Cambrian period, the earliest known examples being two 

 extremely simple forms (Trochocystites and Eocyslites) from 

 the " primordial zone " of North America. Other forms have 

 been described as occurring in the " primordial zone " of 

 Bohemia. In the Chazy and Trenton Limestones of North 

 America, of Llandeilo-Caradoc age (Lower Silurian), and on 

 the same horizon in Eussia, Scandinavia, and Bohemia, 

 Cystideans are found, often in vast numbers, though in a 

 very fragmentary condition. In the Bala Limestone (Lower 



