EAKLY PALEOZOIC BEYOZOA OF THE BALTIC PROVINCES. 229 



In the above diagnosis I have endeavored to give all of the generic 

 features, although it is recognized that some of these pertain equally 

 well to other genera. The massive zoarium of thin-walled, angular 

 zooecia, without acanthopores, occurs in a number of totally unre- 

 lated genera, but these features, in connection with the peculiar 

 glassy-like internal structure, and the occurrence of numerous small 

 granules or tubular structure in the walls, form a combination of 

 characters duplicated in part in only one other group, namely, the 

 peculiar Ordovician genus Nicholsonella. This latter genus has al- 

 ways been regarded as of uncertain affinities because of its unusual 

 internal structure. Its zoaria vary from ramose or flattened branches 

 to free or incrusting expansions externally much like other monticuli- 

 poroids. However, the internal characters of species referred here 

 have always seemed so obscure and ill-defined that, until recently, 

 specimens of Nicholsonella have been regarded as habitually poorly 

 preserved and invariably unsuited for careful microscopic examina- 

 tion. Like Dianulites, its tube walls are thin and in the mature zone, 

 traversed longitudinally by minute tubules which appear at the sur- 

 face and in tangential sections as granules. Acanthopores are absent, 

 the distribution of tubules and mesopores is the same in the two, but, 

 unlike Dianulites, the mesopores in the mature zone of Nicholsonella 

 become entirely closed by a perforated calcareous deposit which ob- 

 scures the characters in this part of the zoarium most effectually. 



Compared with Nicholsonella, Dianulites differs, therefore, mainly 

 in its prevailingly massive zoarium, more simple zooecial structure, 

 and in the absence of a calcareous deposit filling the mature portion of 

 the mesopores. 



Hexaporites was the name given to a condition of Dianulites petropo- 

 litana and has no standing. Further remarks upon this name are 

 given under the discussion of that species. 



DIANULITES FASTIGIATUS Eichwald. 



Plate 2, figs. 1-3; text figs. 127, 128. 



Dianulites fastigiatus Eichwald, Zool. Spec, vol. 1, 1829, p. 181; Letlisea Ros- 

 sica, vol. 1, 1860, p. 488, pi. 28, fig. 9. — Dybowski, Die Chaetetiden der Ost- 

 baltischen Silur-Formation, 1877, p. 20, pi. 1, figs. 1-3. 



Zoarium a turbinate mass with the lower end rounded or truncated, 

 and the upper end, which alone bears the openings of the zooecia, flat 

 or shghtly concave. The sides and lower portion are covered with 

 an epitheca, which, though sometimes smooth, is more often marked 

 by concentric rings or lines of growth. An average example attains 

 the size shown in figure 127 a, but inverted, cone-shaped specimens 

 only a centimeter high have been observed. An entire zoarium is 

 composed of numerous layers of zocecia arranged so evenly upon each 

 other that there often is no break between the zooecial tubes of separate 

 growths. Celluliferous surface smooth; maculae composed of larger 



