74 BULLETIN 16 5, UNITED STATES NATIONAL MUSEUM 



culum. The apertura is suborbicular and hidden at the bottom of 

 a long peristomie; the latter is formed by the frontal avicularia in 

 which the walls are much thickened and by a thick and salient peri- 

 stome; the peristomice is orbicular. 



The ovicell is hidden under the frontal avicularia, resting on the 

 distal zooecium and opening below the operculum. Each frontal 

 is covered by at least 3 small avicularia with thick walls ; 1 is proxi- 

 mal and the other 2 are lateral and open in the vicinity of the 

 peristomices of the adjacent zooecia. Their orifice is small, orbicular 

 without a pivot. Frequently the small proximal avicularium is 

 much developed to form a large avicularium covering all the frontal ; 

 it has a pivot, and its beak is rounded and very salient above the 

 zoarial plane. 



Measuremen ts . — 



rj . fZs = 0.5 mm. ^ . . \hp = 0.1 mm. 



Zooeciai, Annn . , q . Peristomice {/ 



|?s = 0.2-0.25 (?) mm. [lp = 0.1 mm. 



8 zooecia in 1 sq. mm. 



Structure. — A well-prepared longitudinal thin section reveals ad- 

 mirably the structure of this species in particular and of the genus 

 in general. The zooecial walls are thin, but the walls of the small 

 frontal avicularia, on the contrary, are very thick. The latter form 

 the thick carapace that surrounds the colony. There are 2 or 3 on 

 each zooecium. The more inferior (the proximal one of our descrip- 

 tion) is attached to the peristomie of the proximal zooecium and 

 appears to have budded from the latter ; its size is variable and it can 

 transform itself into a large frontal avicularium. The large avicul- 

 arium is a special heterozooecium covering a normal zooecium and 

 arising like it from the proximal zooecium ; a septule (seen in sec- 

 tion) communicates with the subjacent zooecium. 



The ascopore opens in the interior of the zooecium in the close 

 vicinity of the aperture ; it is somewhat curved. 



The ovicell is almost endozooecial, resting somewhat on the distal 

 zooecium, covered by the proximal avicularium of the distal zooec- 

 ium and opening into the same zooecium. The larvae must then 

 have traversed the entire peristomie before escaping into the water, 

 and we can well suppose that the function of the small avicularia 

 surrounding each peristomice is to drive them away and to prevent 

 them from attaching themselves to the colony. 



Variations. — No fixed rule appears to operate in the distribution 

 of the small frontal avicularia. Normally each peristomice is sur- 

 rounded distally by 3 small avicularia, 1 developed on the distal 

 zooecium and the other 2 on the adjacent zooecia; a fourth arising 

 from the same zooecium is sometimes added between the peristomice 



