NORTH AMERICAN EARLY TERTIARY BRYOZOA. 



645 



ADVENTITIOUS TUBES. 



These are ramifications of the polypidian tubes and arise only 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 arc always 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 



25 



FIG. 207. Vacuoles. 



A. Longitudinal thin section, X 25, of Hornera 

 a it tii ret ica Waters, 1904, showing vacuoles on 

 both the frontal (to the left) and dorsal (to the 

 right). B. Longitudinal thin section, X 25, of 

 I'nliiaxcosoccia coronupus, new genus, anil species 

 showing difference between the vacuoles (on the 

 left) and the mesopores (HI) (to the right). 



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 nonabortcd, 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 onlv. 



