262 BULLETIN 100, UNITED STATES NATIONAL MUSEUM 



Variations. — The variations of this species are very great. Al- 

 though much reduced the shield is often entirely visible and bears two 

 oblique triangular avicularia; but when the avicularian umbo is much 

 developed this shield disappears almost entirely with its avicularia. 

 It does not appear to have the function of compensation; but we be- 

 lieve rather that the development of the umbo being correlative with 

 an intense frontal calcification the latter covers the primitive shield. 



The very long spatulate avicularium is attached to the proximal 

 border of the shield; it is rare and appears without apparent reason. 

 The small oral avicularia are lacking very often; they appear irreg- 

 ularly in variable numbers from 1 to 5. This irregularity of the 

 avicularia is discouraging; their presence is certainly not a whim 

 of the animal, but it ought to correspond to some vital necessity. 

 As long as we are ignorant of the function of the avicularium, the 

 biology of the bryozoa can not make great progress. When the 

 avicularia are regular in form and position we can conceive their 

 function to be that of oxygenation (Waters), but in the present case 

 we can not even make a guess. 



The size of the avicularian umbo is very variable; sometimes it is 

 easy to see the lyrula and the cardelles but on other zooecia the umbo 

 hides one half of the aperture. However the neighboring cells al- 

 ways have an umbo equally developed ; never have we observed a cell 

 without umbo at the side of one with a large umbo. 



The size of the radicular pores is rather constant in the neighbor- 

 hood of 0.05 mm. diameter. The perforated area is on the contrary 

 quite variable, even on the same lamella, ranging from 0.15 to 0.35 

 mm. in diameter; it is frequently closed by a chitinous pellicule in- 

 dependent of the ectocyst. 



Affinities. — This species differs from Petraliella falcifera in its large 

 avicularium which is rare and nonfalciform. It differs from Pet- 

 raliella armata Waters, 1913, in its nonfalciform and large oblique 

 avicularium and in its much smaller micrometric measurements. 



Biology. — Without being numerically abundant, this species appear 

 to be well distributed in the Philippines, especially in the south of the 

 Sulu Sea and in its outlet into the China sea. It is insensible to 

 bathymetric variations (34-618 meters) and to the correlative thermal 

 variations. All of our specimens were dead and deprived of radicells. 



Occurrence. — 



D. 5141. Jolo Light, Jolo; 6° 09' N.; 120° 58' E.; 29 fathoms; 



co. S. 

 D. 5144. Jolo Light, Jolo; 6° 05' 50" N.; 121° 02' 15" E.; 19 



fathoms; co. S. 

 D. 5147. Sulade Island, Sulu Archipelago ; 5° 41' 40" N.; 120° 



47' 10" E.; 21 fathoms; co. S., Sh. 

 D. 5162. Tinagta Island, Tawi Tawi Group; 5° 10' N.; 119° 

 47' 30" E.; 230 fathoms; S. brk., Sh. crs.; 11.6° C. 



