NO. 2 OSBURN: eastern pacific BRYOZOA CHEILOSTOMATA 275 



Peristome. The primary peristome is the original fold of the olocyst 

 around the aperture; the secondary peristome develops from the frontal 

 wall and often covers the primary peristome and partially obscures the 

 aperture. 



Pleurocyst. A secondary calcified covering layer which originates at 

 the border of the zooecium and grows toward the center. 



Poster. That portion of the primary aperture proximal to the cardelles. 

 (See sinus.) 



Primary aperture. The original aperture, closed by the operculum 

 which usually fits it very exactly. 



Sclerite. A chitinous thickening of the operculum, either at the margin 

 or otherwise located ; the occlusar muscles are usually attached to it. 



Secondary aperture. The aperture above the level of the operculum, 

 formed by the surrounding frontal wall; it is variable in height and 

 form, complete or incomplete, and is frequently notched on the proximal 

 border to form a secondary sinus or spiramen. 



Shield. A broad, elevated area occasionally surrounding the aperture. 



Sinus. An extension, usually rounded or v-shaped, of the poster into 

 the proximal border of the primary aperture. 



Tremocyst. A secondary calcified layer of the frontal above the olo- 

 cyst; usually thickly perforated all over and developed evenly from the 

 frontal pores instead of growing inward from the border. 



Tremopore. Pores which are scattered more or less evenly over the 

 whole frontal; apparently they all contribute to the formation of the 

 tremocyst ; they are continuations of similarly placed pores in the under- 

 lying olocyst. 



Vestibular arch. A narrow rim surrounding the aperture inside of 

 the primary peristome. 



Umbo. An elevated process or knob-like structure on the frontal 

 usually just proximal to the aperture; occasionally paired, sometimes on 

 the top of the ovicell. 



Zooeciule. A diminutive zooecium sometimes occurring in series with 

 normal ones, usually closed, sometimes bearing an avicularium. 



