Genera of the jSTorth American Palaeozoic Bryozoa. 427 



MATA many forms possess a calcareous plate whose purpose was 

 undoubtedly the same. Fig. 2 illustrates several variations in 

 this organ from the recent cells. 



Fig. 2. 



In the recent cells the opercula are sometimes composed of a 

 continuous chitinous substance, but generally it is composed of a 

 chitinous frame supporting a membrane ; in addition there are 

 sometimes lateral rods and occasionally a complicated frame 

 work. Sometimes the chitinous frame is continuous all around ; 

 at other times the lower border is membranous and is continuous 

 with the chitinous ectocyst of the cell. In other forms the oper- 

 cula are calcified and are preserved in many fossil forms, as in 

 Fenestella, Callopora, Fi:3tulipora, etc. Usually the opercula 

 are somewhat convex, and concentrically striated ; sometimes 

 minutely granulose ; at other times, as in Callopora elegantula, 

 small ridges radiate from the central portion to the margin, hav- 

 ing a resemblance on a superficial examination, to septa or project- 

 ing spinules. Sometimes there is a central perforation, at other 

 times there is a central node or projection. 



The mode in which the opercula are articulated to the cell 

 varies considerably ; when the cells are calcified they are directly 

 attached to the sides of the orifice, usually near the lower border, 

 sometimes above. The articulation is effected by an elastic 

 fibrous ligament, which is generally inserted in a notch on each 

 side of the opercula, but sometimes it is attached to projections 

 which correspond to notches in the margin of the cell. 



The opening and closing of the opercula is effected by two 

 pairs of muscles, the occlusor and retractor, which are variously 

 inserted in the different forms of Bryozoa. 



The opercula are constant in form in the same species, as are 

 also the avicularia, and form the most reliable means for the 

 identification of species in recent forms. Of course in fossil 

 forms, where the opercula are seldom preserved, such a means of 

 identification is impossible. 



