EAELY PALEOZOIC BEYOZOA OF THE BALTIC PROVIlSrCES. 



285 



and with, smaller, subcircular or oval apertures and more or less tMck walls in fully 

 matured examples; arrangement of apertures rather regular in rows about small solid 

 spots, in the immediate vicinity of which the zocecia may be of larger size than else- 

 where; seven to nine in 3 mm. Interspaces apparently sohd and generally with 

 shallow irregular depressions in most specimens, but in very yoimg stages a variable 

 niimber of irregular mesopores may be recognized. Acanthopores nmnerous, two or 

 more to each zooecium, situated in the angles of junction and interspaces, and increas- 

 ing in size with age. They are large and a conspicuous external feature of well-pre- 

 served mature examples. 



^ 



'isst-^ 



Fig. 173. — HEMIPmiAGMA IEBASTJM. a, view, natural size, of the LAEGE, lEEEGirLABLT BRANCHING 

 BASAL PORTION OF A ZOARITJM; 6, TANGENTIAL SECTION OF THE SAME SPECIMEN, X20; C, SEVERAL ZOCE- 

 CIA, X35, ILLUSTRATING THE MINUTE WALL STRUCTURE; d VERTICAL SECTION, X20, PASSING THROUGH 

 BOTH REGIONS. ORTHOCERAS LIMESTONE (B3), BaLTISCHPORT, ESTHONIA. 



Internal characters. — In the axial region of vertical sections the tubes have thin and 

 irregularly fluctuating walls and few or no diaphragms. The latter are complete here 

 and the proximal end of the tube expands to full size with unusual rapidity. In the 

 peripheral region, which is naiTow and abruptly distinguished from the axial, the 

 walls are more or less thickened, and the tubes intersected by semidiaphragms, 

 about foup in 0.5 mm. I have satisfied myself that all the transverse partitions in 

 this outer part of the zocecial tubes are really incomplete. That many may appear 

 entire in sections is only because their inner edge happens to be vertical instead of 

 horizontal. Mesopores are difficult to make out in these sections, being short and 

 usually filled, in part at least, with solid tissue. Tangential sections require no descrip- 

 tion, all the essential characters being shown in the figures. 



