260 



BULLETIN 77, UNITED STATES NATIONAL MUSEUM. 



ESTHONIOPORA COMMUNIS, new species. 



Text figs. 151-155. 



Zoarium of regular dome-shaped or hemispheric masses, with a flat, 

 concentrically wrinkled base; growth beginning upon some foreign 

 object, which, in the mature stages, is overgrown or is lost and 

 usually indicated only by the cicatrix of attachment. A well devel- 

 oped zoarium is 30 mm. in its basal diameter and 20 mm. in height, 

 but all sizes from this or even larger, down to masses less than 10 mm. 

 wide, are found. The usual form of zoarium is shown in figure 152 a, h, 

 but a less usual occurrence is illustrated in figure 155. Here the 

 zooecia of one portion of a normally growing example have continued 

 growth and have given rise to another normal but attached form. 



Fig. 151.— ESTHONIOPOEA COMMUNIS, a 'and 6, TWO VERTICAL SECTIONS, X20, SHOWING AN IREEGULAE 

 AEEANGEMENT OF THE SEMIDIAPHEAGMS. KUCKERS SHALE (02), BARON TOLL'S ESTATE, ESTHONIA. 



Surface smooth, maculae 4 to 5 mm. apart, inconspicuous to the un- 

 aided eye, and distinguished under a hand lens or in sections by 

 clusters of zooecia larger than the ordinary. Zooecia thin-walled, 

 polygonal, usually hexagonal, with three and one-half to four in the 

 space of 2 mm., measuring from the center of a macula. An ordi- 

 nary intermacular zooecium is 0.5 mm. in diameter. Mesopores and 

 acanthopores wanting. The bases of some zoaria are covered with 

 small, mesopore-like cells sometimes noticed in other massive 

 bryozoans. 



Tangential sections show exceedingly simple zooecia with the thin 

 walls of those adjacent to each other amalgamated. No trace of 

 acanthopores or mesopores is visible, although small, mesopore-like 

 spaces representing young cells are sometimes present. In vertical 

 sections the presence of semidiaphragms at distances averaging a tube 

 diameter apart is the most important character to be noted. Fre- 



