S UB- GENUS PR A SOPOR A . 



205 



series of large circumferential vesicles, the convexities of which 

 are directed upwards and towards the centre of the corallite. 



F'g- 43- — Minute structure of Prasopora Graytc, Nich. and Eth. jun. , from the Lower 

 Silurian of Girvan, Ayrshire. A, Part of a tangential section, enlarged eighteen times, 

 showing the two sets of corallites, and the singular perforated tabula; of the large tubes ; 

 B, Part of a vertical section, similarly enlarged, showing the entirely different nature of 

 the tabulation in the two sets of corallites respectively. 



When the section does not pass accurately through the median 

 plane of the corallites (as it very commonly does not), then it 

 comes to intersect the exterior tabulate zone, and the cut edo-es 

 of the vesicular tabular appear as transverse lines and simulate 

 ordinary tabular ; so that in most sections parts of the tubes 

 exhibit one set of appearances, and parts show . the other. 

 When examined in tangential sections (fig. 43, a), the large 

 corallites are seen to be hexagonal, prismatic, or sub-polygonal, 

 and in the centre of each is a rounded or oval opening repre- 

 senting the transverse section of the central tube of the coral- 

 lite. This opening is surrounded by a variable number of 

 curved lines, which are tangents to the margin of the median 

 aperture, or are concentric with it, or intersect one another. 

 These lines are the cut edges of the vesicular tabulse which 

 form the exterior zone of the corallite ; and when thus exhib- 

 ited they closely resemble the similar lines produced by the 

 transverse section of the central tube and the infundibuliform 

 tabulae in Syringopora. 



The smaller corallites of the colony are seen in tangential 

 sections (fig. 43, a) to be wedged in among the large tubes, 



