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BULLETIN 106, UNITED STATES NATIONAL MUSEUM. 
crowded on the large walls (fig. 4, A). When there is a mucro, the elements are 
grouped in radial rows (fig. 4. B). The walls of the mural rims in the Mala- 
costega also have their elements grouped in rows perpendicular to the lines of 
the zooecial sutures (fig. 4, C). The olocystal walls are always smooth (fig. 4, D ) ; 
they are perforated by very small pores when they are surmounted by a tremocyst 
or a pleurocyst. 
Tremocyst . — -The frontal endocvstal buds deposit above the olocyst a second 
calcareous deposit called the tremocyst ; the pores thus formed are the tremopores ; 
Ax ioo Bxioo Cxioo 6 x 20 
Fig. 4. — Structure of the olocyst. 
A. Thin section through the basal wall of Membraniporidra spissimuralis, new species, X 100, 
showing olocyst with crowded elements. 
B. Olocyst of Acanthi onell a oecioporosa, new species, X 100, with elements grouped in radial 
lines. 
C. Thin section of Periporosella tantilla Canu and Bassler. 1917. X 100, exhibiting elements 
of olocyst grouped in series around the mural rim. 
D. View of interior of Smittina angulata lie uss, 1865, X 20. showing the smooth olocyst, per- 
forated by areolae only. 
the zooecia, which are provided with them, are tremogastres ; the tremopores are 
funnel shaped or tubular; they are always traversed by mesenchvmatous elements. 
The funnel-shaped tremopores arise from a very small pore perforating the 
subjacent olocyst (fig. 5, A) ; they become enlarged at the exterior (fig. 5, B ). 
The tubular tremopores or tubulae also arise from a small pore piercing the 
subjacent olocyst (fig. 5, O ) , but the latter is very thin and permits one to see through 
a circular, white area the base of the tubula itself. At the exterior the tubules have 
the same diameter (fig. 5, D). 
Often the tubular pores are united among themselves and the number of the 
interior pores (fig. 5, E) does not correspond to the number of exterior pores (fig. 
