io8 



NORTH AMERICAN INDEX FOSSILS. 



are twisted in a spiral form, of which generally the solid axis alone 

 is fully preserved in fossil form. 



In the Palaeozoic genera the cell apertures are often surrounded 

 by elevated rims or peristomes. Often a portion of the poste- 

 rior wall is more or less thickened 

 and curved to a shorter radius, form- 

 ing lunaria, the ends of which may 

 project into the tube as pseudosepta. 

 At the junction of the apertures 

 small cylindrical tubules projecting 

 above as spines, the acanthopores, of- 

 ten occur. When the zocecial tubes 

 are crowded and thin-walled their 

 apertures generally have an angular 

 outline, but when they are thick- 

 walled or separated by interspaces, 

 they are circular, oval or variously 

 formed. The interspaces may be oc- 

 cupied by smaller tubes, the meso- 

 pores, or by vesicular tissue. The 

 mesopores may be scattered or gath- 

 ered into clusters. Elevations or 

 monticules and flattened or depressed 

 spots or maculae are other charac- 

 teristic surface features. They may- 

 be solid or contain the openings of 

 zocecia (generally somewhat larger than the ordinary) or of 

 mesopores. 



In the Cryptostomata the opening of the zocecium is the cell 

 orifice. Above this occurs a tubular shaft formed by the thicken- 

 ing of the surface of the zoarium for strengthening or protective 

 purposes. This shaft is the vestibule, and its aperture is variously 

 formed. 



In most Palaeozoic Bryozoa the tubes are variously divided by 

 cross plates or diaphragms which in some types are vesicular, form- 

 ing cystiphragms (Trepostomata). The diaphragms may be com- 

 plete or pierced by a central opening. In the Cryptostomata addi- 

 tional plates the hemisepta project from the wall into the cavity. 

 When they project from the posterior wall, they are known as 

 superior hemisepta, when they project from the anterior wall they 



Fig. 176^. Diagram showing 

 structure of single bryozoon zooid. 

 (After Busk.) a, anus ; c, ectocyst; 

 d, perigastric space ; g, nerve gang- 

 lion ; i, intestine ; /, lophophore ; 

 m, month ; 0, ovary ; oc, oesopha- 

 gus ; or, aperture of the zocecium ; 

 r, retractor muscles ; s, stomach ; 

 /, tentacles ; v, tentacular sheath ; 

 x, testis ; 2, funiculus. 



