264 PROCEEDINGS OF THE NATIONAL MUSEUM vol. los 



Avicularia: An internal avicularium is present but does not show 

 from the outside because it is placed low and obliquely, a bit to the 

 side, as in plate 15,D. It has a hooked beak and triangular mandible, 

 which also is hooked at the tip. The internal avicularium is 0.238 

 mm. long and 0.144 mm. wide, based on one reading. Its beak is 

 about 0.17 mm. long. 



External avicularia occur in pairs generally. However, an occa- 

 sional zoid is found which has a single external avicularium at the 

 usual location. Mucros are absent. An external avicularium leans 

 against each lower corner just below the primary orifice. The avic- 

 ularium is bent so that the beak and the back area are in different 

 planes. This is most accurately shown in only one drawing (pi. 

 16,A, lower row of zoids, second avicularium from the left), although 

 it is the common condition. The external avicularium normally points 

 obliquely upward and outward. Its beak and mandible are hooked 

 and triangular (pi. 16,A,B). External avicularium measurements: 

 total length 0.158-0.202 mm. (average 0.171 mm.); total width 

 0.115-0.137 mm. (average 0.121 mm.); mandible length 0.072-0.101 

 mm. (average 0.088 mm), width 0.101-0.122 mm. (average 0.109 mm.). 



Frontal oral ledge. The frontal oral ledge is parallel to the 

 plane of the orifice (pis. 15,D,E,G; 16,C,I). Plate 16, D shows a 

 young orifice that has not yet developed the ledge. 



Orifice: The young, just-fonning primary orifice is nearly hemi- 

 spherical (pi. 16,D). A fully formed primary orifice, complete with 

 oral ledge as its proximal boundary, is crescentic (pis. 15, D; 16,0). 

 As calcification of the frontal wall proceeds, the primary orifice is 

 deeply buried and can no longer be fully studied from the front 

 (pis. 15,G; 16,A). There is a gradual overgrowth that eventually 

 results in a secondary orifice whose distal wall is either straight or 

 arched gently forward and whose sides lead down to the external 

 avicularia, thus enclosing an area shaped like an inverted triangle 

 (pi. 16,A). The upper surface of the overhanging upper edge may be 

 tubercled. Dimensions of the orifices: primary orifice length 0.130- 

 0.187 mm. (average 0.156 mm.), width 0.245-0.317 mm. (average 

 0.284 mm.); secondary orifice length 0.144-0.245 mm. (average 0.190 

 mm.), width, 0.202-0.360 mm. (average 0.301 mm.). 



Ovicells: Present on some zooecia, not on others. It is difficult to 

 tell externally in heavily calcified pieces whether an ovicell is present, 

 but it is easy enough in very young zoids. Where the frontal wall 

 has not become too thick the ovicell outlines are barely visible (pi. 

 15,E). In older zoids the frontal over the ovicells becomes thick 

 (pi. 15,G) and they are harder to recognize. They are covered by a 

 secondary porous or channeled layer but their own inner wall is not 



