EARLY PALEOZOIC BRYOZOA OF THE BALTIC PROVINCES. 191 



are conspicuous features of these sections, but their relative abundance varies some- 

 what in different examples. In the axial region of vertical sections the walls of the 

 tubes are very thin and finely wavy, and the diaphragms straight and remote, or 

 wanting entirely. Aa the tubes enter the peripheral region the number of diaphragms 

 is greatly increased, the walls thickened, and cystiphragms, mesopores, and acan- 

 thopores developed. The mesopores are distinguished fi-om the zooecia by their 

 shortness and in having no cystiphragms. The latter structures number from three 

 or fom- to fifteen in a direct series in each zooecial tube. In most cases they occur 

 only in the region intervening between the fully matured peripheral and the imma- 

 ture axial region. Beyond them the diaphragms are crowded and essentially hori- 

 zontal. In the mesopores the diaphragms are often thick and situated about the same 

 distances apart as in the zocecial tubes, with from fourteen to seventeen in 1 mm, In 

 the axial region of transverse sections the zooecial tubes are of unequal sizes and of 

 peculiarly irregular shapes. 



d 



Fig. 102.— Homotrypella instabilis. a, 6, and c, veetical sections, x20, showing biffeeent stages 

 in the development of the mature zone; d, tangential section, x20. kuckers shale (c2), 

 Baeon Toll's estate, Esthonia. 



Many of the Russian examples agree with the above description, 

 but a few of them are young examples which have furnished, the thin 

 sections illustrated in figure 102. In figure 102 a, the vertical sec- 

 tion of a stage so youthful that the mature region is scarcely devel- 

 oped, is shown. Distinct cystiphragms are noted in the bend to the 

 mature zone but they are incomplete at their lower end. In figure 

 102 6, a shght advance in the development of cystiphragms may be 

 seen, and a further advanced stage, with the introduction of tabu- 

 lated mesopores, is illustrated in figure 102 c. In all of these vertical 

 sections it is evident that only the earhest cystiphragms are incom- 

 plete. Possibly their lower portion consisted of tissue too delicate 

 to show in thin sections. In older zooecia most of the early cysti- 



