54 BULLETIN 16 5, UNITED STATES NATIONAL MUSEUM 



The avicularia are oftenest grouped around the aperture without 

 fixed place ; there are rarely two of them and frequently none at all. 

 Their orientation is variable. The primoserial avicularia are trans- 

 verse; the others are oriented proximally. 



On the interior of the small zoarial tubes the zooecia are visible 

 and distinct ; they all bear a distinct distal tuberosity. These tuber- 

 osities separate the rigid basal lamella from the flexible substratum ; 

 they glide over it and thus prevent the rupture of cells. 



The aperture having constant dimensions, the frontal is covered 

 by the ectocyst, and the opercular valve is much chitinized. As the 

 aperture of the ovicelled zooecia has the form and the dimensions 

 of the others, the operculum closes the ovicell. 



Occurrence. — Vincentown limesand : Rare at Vincentown, N.J. 



Cotypes.— V.S.N. M. No. 73915. 



RHINIOPORA PARVIROSTRATA, new species 



Plate 12, Figure 1 



Description. — The zoarium encrusts fragments of shells. The 

 zooecia are distinct, separated by a furrow, elongated, elliptical, 

 rather large, swollen; the frontal is convex and formed of 16 to 18 

 flat, radiating costules much crowded and separated by rows of 

 minute vacuoles. The aperture is terminal, semielliptical, trans- 

 verse ; the apertural bar is distinct, separated from the peristome by 

 a furrow ; it is smooth and of the same nature as the costules. The 

 distal peristome is thin, smooth, salient, ornamented with two short 

 spines. The ovicell is hyperstomial, closed by the operculum, em- 

 bedded in the distal zooecium, large, globular, smooth. In many of 

 the interzooecial angles there is a very small avicularium, triangular, 

 without a pivot indifferently oriented. 



Measurements. — 



. \Lz = 0.6-0.7 mm. . ik=0.11mm. 



Zooecia , _ OK A . Apertura 



\lz = 0.35-0.4 mm. [te=0.15 mm. 



16 zooecia in 4 sq. mm. 



Affinities. — The structure is absolutely identical with that of Rhi- 

 niopora tubulosa, but R. parvirostrata differs in its smaller micro- 

 metric measurements, in its narrower rows of vacuoles, in the rela- 

 tively much smaller apertural bar, and above all in the extreme 

 minuteness of the avicularia. One might naturally ask what is the 

 use of such small avicularia for such large cells. 



Occurrence. — Vincentown limesand : Very rare at Noxontown 

 Millpond, Del. 



Cotypes.—U.S.'NM. No. 73916. 



