428 



Eepoet 01 THE State Geologist. 



The form in recent species varies, but they are all more or less 

 circular or semicircular in outline. The lower border is straight, 

 sinuate or concave, forming a segment of a smaller circle than 

 the upper border, or produced in the middle into a peduncular 

 process, which usually, but not always, corresponds to a notch in 

 the margin of the cell. 



AviCULARIA AND YlBRACULA. 



The chitinous organs called avicularia were first observed by 

 Ellis in a species to which he gave the name "bird's head coralline," 

 and it is from their resemblance to the beak of a bird that they 

 take their name. They are supposed to be modifications of the 



Figs. 3-5. Illustrating immersed avicularia. 

 Fig. 3. Cellepora Honoluluensis. 

 Fig. 4. Cellepora vagans. 

 Fig. 5. An enlargement of the mandible of the latter (after Busk). 



cell proper. There are three distinct forms, the simplest of 

 which is' in the rudimentary form of a dwarfed cell, with an 

 enlarged operculum, called the mandible (figs. 3, 4, 5), and is 

 known as the immersed form. The SQSsile forms (figs. 6\ Y, 8)^ 

 are those which are situated on the cell walls and have a small 

 chamber and mandible. The pedunculate forms, which are situ- 

 ated at the extremity of a movable stalk, which is frequently 

 jointed, and which is in almost incessant motion (figs. 6'' and 9). 

 These avicularia have a very close resemblance to a bird's head. 

 All the forms may be said to consist of three parts, the chamber, 

 the beak and the mandible. In the chamber are occlusor and 

 retractor muscles, by means of which the mandible keeps up a 

 constant flapping motion. The different parts and the muscles 

 are illustrated in figs. 9, 10, enlargements of sessile avicularia. 



