NORTH AMERICAN EARLY TERTIARY BRYOZOA. 



801 



Meas urements, 



ornamented with large vacuoles. The peristome is salient, thin, elliptical, or oval. 

 The dorsal is very porous; the longitudinal sulci are of little depth, the nervi 

 are wide but little convex; the vacuoles are large but of lesser dimensions than 

 the frontal vacuoles. 



Diameter of the peristome 0.10 mm. 



Diameter of the aperture 0.06-O.OS mm. 



Distance between the peristomes 0.50 mm. 



Separation of the peristomes O.^i mm. 



Variations. The peristome is very salient (fig. 8) or scarcely projecting (fig. 

 10). When the frontal vacuoles are smaller (fig. 10) they are more numerous. 

 The dorsal is exceedingly variable and it is impossible to discover the rule of varia- 

 tion. However, on the J 7 oung branches the. sulci are very narrow and the nervi 

 very wide (fig. 14). Somewhat later the vacuoles are more visible (figs. 13, 15). 

 Normally (fig. 12) the vacuoles are large and the nervi little salient. 



D -.'5 



FIG. 258. Genus Crassohornera Waters, 1887. 



A, B. Frontal, X 12, and dorsal, X 8, of Crassohornera (Ceriopora) arbuscula Reuss, the 

 latter showing the ovicell. 



C-E. Views of the zoarium, natural size, and the frontal and dorsal, X 25, of Crassohornera. 

 traipuktire-nsis Waters, 18S7. 



The dorsal vacuoles are enlarged at their extremity, for in tangential sections 

 they appear smaller (fig. 17). As in all the species of the genus the elements are 

 thick and crowded which makes it very difficult to obtain good thin sections 

 (fig. 19). 



The squamous and lamellar structure of the walls appear to result from the 

 linear union of the primitive elements (fig. 18). 



Affinities. The species singularly resembles Hornera frondiculata Lamou- 

 roux, 1821, in the number of frontal vacuoles and in the elliptical form of the 

 peristomes. It differs from it, however, in a smaller number of longitudinal sulci 



55890 20 Bull. 106 51 



