NORTH AMERICAN EARLY TERTIARY BRYOZOA. 
645 
ADVENTITIOUS TUBES. 
These are ramifications of the polypidian tubes and arise onty on the frontal 
part of the latter. This difference is fundamental and permits no confusion. The 
adventitious tubes are classed as vacuoles and mesopores. 
Vacuoles . — The vacuoles are parietal perforations with nonadjacent walls be- 
tween them. They open at the base of longitudinal furrows called sulci and bend 
outward at a right angle. Vacuoles seem to characterize the family Horneridae 
(fig. 207), although they have been noted in at least one other family (Ascosoe- 
ciidae) . 
Mesopores . — The mesopores are 
superior and cylindrical ramifications 
of the bent tubes; they are without 
polypide and are ahvays parallel to the 
superior part of the tubes. In the 
club-shaped zooecia their walls are 
generally simple (= maculae, cancelli, 
of Gregory) (fig. 208 C), but on cy- 
lindrical zooecia their walls are usu- 
ally vesicular (fig. 208 A). However, 
there are numerous exceptions to this. 
Mesopores are almost always of 
smaller diameter than the generative 
tubes; they seem to be almost always 
closed by a very fragile calcareous 
lamella little resistant to fossilization. 
and finally they may branch among 
themselves (fig. 208 D). 
Ulrich, the author of the word 
mesopores , defined them in 1890 as 
“ angular or irregular cells occupying 
interzooecial spaces in certain Paleo- 
zoic genera.” The accessory tubes, like 
the adventitious tubes, are included in this definition in spite of their difference in 
origin and probably function. In 1896 Gregory defined them more precisely as 
“ aborted zooecia, which are smaller in diameter than the normal zooecia,” and 
in 1899 as “ rudimentary zooecia.” 
Aborted or nonaborted, a zooecium is a zooecium; it should have the same 
origin as a polypidian zooecium and should grow from another zooecium by a 
special mode of gemmation and before its complete calcification. Any cellular 
cavity not having this origin is not a zooecium (=tube) but is only a ramification. 
This consideration of origin obliges us therefore to change the nomenclature some- 
what. Two solutions are possible, first, to preserve Ulrich’s definition and apply 
the term “ mesopore ” to all structures which are not polypidian tubes in conform- 
ity with the ideas of the author, or second, to restrict it to the zooecial ramifica- 
tions only. 
-Vacuoles. 
A. Longitudinal thin section, X 25, of Eornera 
antarctica Waters, 1904, showing vacuoles on 
both the frontal (to the left) and dorsal (to the 
right). B. Longitudinal thin section, X 25, of 
Polyascosoecia coronopus, new genus, and species 
showing difference between the vacuoles (on the 
left) and the mesopores (m) (to the right). 
