140 BULLETIISr 77, UNITED STATES NATIO^-AL. MUSEUM. 



PACHYDICTYA FLABELLUM (Leuchtenberg). 



Plate 8, fig. 1; text figs. 63, 64. 



Eschara flabellum Leuchtenberg, Geognosie de Russie, p. 370. 

 Stictopora flabellum Eichwald, Lethgea Rossica, vol. 1, 1860, p. 391, pi. 24, 

 fig. 14 a-c. 



The particular bryozoan that Leuchtenberg had in mind when 

 describing Eschara fldbellum can probably never be determined, and 

 our knowledge of this species must date from Eichwald's work. 

 Although Eichwald's description leaves much to be desired, his 

 figures agree exactly, so far as they go, with an abundant bryozoan 

 of the Wesenberg limestone belonging to the genus Pachydictya. I 

 have figured this species in some detail, and will call attention to 

 the various illustrations in giving its specific characters. 



As shown in figure 1 of plate 8, the 

 zoarium is of thin, bifoliate, ribbon-like, 

 graceful branches, less than 5 mm. 

 wide, dividing dichotomously at inter- 

 vals of 15 mm. The margins are acute 

 and usually have a finely striated bor- 

 der. The width of these marginal bor- 

 ders increases with age, as illustrated 

 in Eichwald's view of the surface (fig. 

 63 &). The zocEcial apertures are oval 

 with a poorly developed ''ring" or 

 peristome which is usually inconspicu- 

 ous because the lines of theinterzooecial 

 spaces unite to form a hexagonal area 

 about each aperture. This, as is evi- 

 dent in Eichwald's figure, gives an ap- 

 parent polygonal outline to the zooecia, and, moreover, makes the 

 species easy of recognition. Measuring longitudinally, five* to six 

 apertures occur in 2 mm. 



The internal structure is much as in other species of Pachydictya 

 of similar growth. The vertical sections show unusually large 

 vesicles fiUing the early portions of the interzooecial spaces, but these 

 are soon replaced by the sohd, laminated tissue pierced by the minute 

 tubules. Tangential sections passing near the surface of old ex- 

 amples exhibit oval zooecia with a narrow peristome, separated by 

 broad, interzooecial spaces bearing numerous tubules (fig. 64 a). In 

 figure 64 5 a deeper portion of a tangential section is illustrated with 

 the lower half passing through the vesicular zone. In certain sec- 

 tions, especially of young examples, the vesicles have the appearance 

 of mesopores (fig. 64 c). 



The only species with which P. jidbeUum may be confused is the 

 equally abundant P. elegans. Comparisons between the two are 

 given under the description of the latter. 



1 



Fig. 63.— Pachydictya FLABELLUM. a, 



FRAGMENT, NATURAL SIZE; 6, SURFACE OF 

 SAME, ENLARGED, SHOWING THE SOME- 

 WHAT ANGULAR ZOOECIA AND PORIFEROUS, 

 NONCELLULIFEROUS BORDERS; C, TRANS- 

 VERSE VIEW OF BRANCH. "CALCAIRE 1 

 ORTHOCERATITES," PULKOWA, GOVERN- 

 MENT OF St. Petersberg. (Copied 

 FROM Eichwald's figure of Sticto- 

 pora FLABELLUM.) 



