NORTH AMERICAN EARLY TERTIARY BRYOZOA. 



51 



The calcareous secretion is first active about the areola (fig. 8, .4). but it extends 

 rapidly over all the frontal. 



The pleurocystal deposits are often granular (fig. 8, B] but they generally form 

 interareolar costules (fig. 8, C) so frequent in Smittina, Hippomenella, etc. 



The porous pleurocyst is more rare (fig. 8. D, E) the pores are unequal, irregu- 

 lar, and arise from the irregular union of the primitive granulations. Contrary 

 to the tremopores they do not perforate the subjacent olocyst (fig. 8, D). 



The pleurocyst is easily detached from the olocyst (fig. 8. F). 



The areolae appear clearly in the .tangential sections when made at the level 

 of the olocyst (fig. 8, G). Made on the exterior plane the interareolar costules alone 

 appear (fig. S, H). 



4m,-- {-- 



FIG. 7. Structure of the tubules. 



A-C. Myriozoum subgracile D'Orbigny, 1852. A. Section, X S5, through polypide showing the 

 disks (d) in the pore tubes, diaphragm (dm), and the operculum (op), with dotted lines to show 

 the position when partly open. (After Waters, 1900.) 



B. End of tubula, X 500, showing the covering integument a (ectocyst) and the inner one, b. 

 (After Waters.) 



C. Each of the tubes contains a mesenchymatous tractus, and is, at its proximal part, fur- 

 nished with a uniporous septula. (After Levinsen. 1909.) 



Made in an intermediate plane they show some radial reticulations which are 

 evidently the intersections of the different calcifications (fig. 8, /). 



On recent bryozoa these three fundamental deposits are subject to some inter- 

 esting variations, the study of which was only begun in 1909 by Levinsen. 1 Wo 

 refer the reader to his work, for on the fossils these variations are quite invisible. 



Septulae and Dietellae. The mesenchymatous fibers pa>.s from one zooecium to 

 another by small parietal pores called septulae. The septulae are uniporous (fig. 

 9, A) or multiporous (fig. 9, B) (=rosette-plates of authors). On the fossils the 

 septulae are rarely visible. The multiporous septular disks leave a large pore 

 often visible exteriorly, as in the Membraniporae. For shortness in this mono- 

 graph we called these structures also septulae. 



1 1009. Levinsen, Morphological and Systematic Studies on the Cheilostomatous Bryozoa, pp. 1-7. 



