64 Cumings — Development of some Paleozoic Bryozoa. 



Morphology of the Oarince. — The primary carinae first make 

 their appearance in the metanepiastic stage, and are intimately 

 related to the basal plate (figs. 47-54). In fact the carina 

 seems to originate as an upgrowth or fold of the basal plate. 

 It is structurally double or more properly triple (figs. .57-62), 

 consisting of a thin median plate or wall continuous with the 

 axial wall (w) and coinciding with planes of division between 

 adjacent zooecia ; and on either side an outer layer of dense, 

 punctate sclerenchyma. This median or proper wall, as it 

 approaches the axis, becomes extremely thin, and is therefore 

 apt to be destroyed, a fact that will account for the hiatus 

 between d and f and the stalk of the zoarium in figs. 48 and 49. 

 In early stages the carinae are discrete, except near their junc- 

 tion with the basal plate, where they coalesce with the 

 upturned rim of the latter (figs. 47, 51) ; but in ephebastic 

 and gerontastic stages the earlier formed zooecia become 

 entirely submerged in a copious deposit of secondary scleren- 

 chyma, which bridges adjacent carinae, and gives a perfectly 

 smooth cylindrical aspect to the stalk of the colony (cf. figs. 

 52, 58 and 60). 



Figures 61, 62. — Transverse sections of base of Fenestella from the Hamil- 

 ton formation of Thedford, Ontario. 61. Transverse section in the plane 

 of b', fig. 55, showing the presence of five primary carina? and ten zocecia. 

 The heavy external wall joining a, b, c, d, e, is of secondary origin. 

 In early stages the carina? a, 6, c, d, e, are discrete. The axis, ax, has a well- 

 defined wall, xv, continuous with the primary axes of the carinae. 62. 

 Same section as fig. 58 — axial portion showing axial wall, w, and its continua- 

 tion as the axes or midribs of the carina? ; a-e, carinae ; 1-10, zooecia. 



Figure 61 x 28 ; figure 62 x 31. 



The height of the carinae, breadth of the basal plate, and 

 position and extent of the secondary deposits are sharply 

 defined by the limits of action of the exserted polypides. 

 The circular or semicircular depressions frequently seen in the 



