642 
BULLETIN 106, UNITED STATES NATIONAL MUSEUM. 
what oblique, and club-shaped, and as short as it is distant from the polypidian 
tubes; second, of a large, transverse, and cylindrical part. The orifice on the 
dorsal of the zoarium is polygonal and on fossils always open. 
When a tergopore is aborted (fig. 202 A), it is immediately replaced by two 
others, formed by the simultaneous bifurcation of the distal and proximal tergo- 
pores. 
A. Longitudinal thin section of Mesonea suhper- 
tusa } new species, X 25, illustrating structure of ter- 
gopores (to right) and zooecial tubes. B. Transverse 
section, X 25, of the same species showing the 
zooecial tubes iu the upper half and the tergopores 
below. C. Broken branch of same, X 12, with the 
layer of the tergopores separated from the zooecial 
tubes. D. Dorsal of same, X 12, exhibiting large 
tergopores. 
Longitudinal thin sec- 
tion of Idmidronea coro- 
nopus Milne Edwards, 
1S38, illustrating struc- 
ture of firmatopores (to 
right). 
The ectocyst of the tergopores was much thicker than that of the polypidian 
tubes, for its disappearance shows a much larger, interzooecial space (fig. 202 A). 
As the tergopores give rise to one another independently of the polypidian 
tubes, their ensemble is easily separated from the frontal of the zoarium (fig. 
202 C). They evidently form part of the system of basal fixation of the zoarium. 
In sections, they have the appearance of mesopores but they differ in their forma- 
tion for these are tubes and not ramifications. 
