430 



Eefoet of the State Geologist. 



Hi 



triangular, elongate or sword shaped. They are formed of a 

 strong chitinous frame and base, by the extremities of which the 

 mandible is articulated to the cup. The space between the frame 

 is occupied by a membrane, nearly always with a foramen, above 

 which the occlusor muscles are inserted. These are usually two 

 in number. The form of the retentive mandible is usually semi- 

 circular or spatulate. They are generally simple, but sometimes 

 bifid or trifid. They are without the continuous chitinous frames 

 of the prehensile mandibles, and consist of a membrane sup- 

 ported only by the base, or by a frame at the sides extending 

 only a short distance from the base ; the^foramen is generally 

 absent, and the occlusor muscles are much weaker than in 

 the other variety. The purpose of the avicularia is not 

 known. By some it is thought that they procure food, 

 and it is a favorite mode of illustration to picture an avicu- 



FiG. 8. Menipea flagellifera. a, Sessile avicularia ; v, Vibracula ; p. Peristome ; o, Opercula. 

 Fig, 9. Bugula bicornis, showing two kinds of pedunculate avicularia, the larger one with the 

 dlgitiform process (after Busk). 



