Genera of the ISTorth American Palaeozoic Bryozoa. 429 



In the Report on the Bryozoa of the Challenger expedition, 

 B L'8K arranges the avicularia as follows : 



As to form : 



a. Pedunculate and usually articulate. 

 1). Sessile. 

 c. Immersed. 

 As to function : 



a. Prehensile — when the mandible, beak and muscles are 

 adapted for prehensile purposes. 



h. Retentive — when the mandible is thin, membranous, 

 and adapted to serve merely as a lid for the cup or receptacle. 

 As to position : 



a. Vicarious — when they represent or replace an ordinary 

 zooecium. 



5. Adventitious — when either attached to some point or 

 other of the zooecium or interspersed among the zooecia. 

 The mandibles exhibit a great variety of form, some of which 

 are shown in figure 12. The prehensile forms are semicircular. 



Fia. 6'. Sessile avicularia of Scrupocellaria scruposa. a, Mandible; b, Beak; c. Chamber of avi. 

 cularium; m. Muscles ; p, Peduncle, 

 f Fig. 6". Pedunculate avicularium, ideal figure. 



Fig. 6'". Tliree cells, showing vibracula. 



Fig. 7. Cell of Eschara sulcata, with sessile avicularia (after Hixcks and Busk). 



