132 



ANNULOIDA. 



the " pyramid " must be the anus. There is, however, in some 

 cases a third aperture of small size, always placed near the 

 apex, and this has been regarded as being truly the anus. 



Many Cystideans were further provided with a system of 

 minute pores or fissures, penetrating the plates of the calyx, 

 and usually arranged in definite groups. These groups are 

 termed " pectinated rhombs " (fig. 84), and their exact func- 

 tion is doubtful. By Messrs Billings and Rose, however, they 

 are believed, apparently with good reason, to have admitted 

 water to the body-cavity, and to have thereby subserved a 

 respiratory function. 



r> 



Fig. 84. A, Pectinated rhomb of Glyptocystites multiporus (Billings). B, Pectinated 

 rhomb of Pletirocystites (Billings). C, Two plates of Callocystites Jewetti (Hall), show- 

 ing the pectinated rhombs (/). All enlarged. 



As regards the distribution of the Cystideans in time, they 

 are not only wholly extinct, but they are exclusively confined 

 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 Eocystites] from the "primor- 

 dial zone" of North America. Other forms have been de- 

 scribed 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 Russia, Scandinavia, and Bohemia, Cystideans are 

 found, often in vast numbers, though in a very fragmentary 

 condition. In the Bala Limestone (Lower Silurian) of Britain, 

 Cystideans are abundant fossils. In the Upper Silurian (Nia- 

 gara and Lower Helderberg) of North America, and on the 

 same horizon (Wenlock and Ludlow) of Britain, Cystideans 

 still occur, though their remains are not so plentiful. Lastly, 

 in the Devonian Rocks occur forms which have been doubt- 

 fully referred to Cystideans, but the real nature of which is 

 uncertain. Upon the whole, then, the Cystideans appear to 

 have commenced in the Upper Cambrian Rocks, to have 



