102 BRITISH STROMATOPOROIDS. 



or vesicular tabulae, and which gives off diverging lateral canals which are also 

 tabulate. The general skeletal tissue is built up in concentric layers round the 

 main axial tube (Fig. 15), the concentric laminae and in terlaminar spaces being thus 

 very conspicuous. On the other hand, the radial pillars are proportionately much 

 less developed than in I. Boemeri, while the small zooidal tubes are tortuous and 



a 



Fig. 15. — Transverse section of half of a stem of Idiostroma oculatum, n. sp., enlarged 

 twelve times, a. The axial canal, transversely divided, b b. The large radial tubes, 

 longitudinally divided, opening inferiorly into the interlaminar spaces, but acquiring 

 thickened walls, and being intersected by tabula? where they approach the surface. 



irregular. Some specimens show no further character which would demand notice 

 here, the surface being simply covered with small tubercles, often arranged in 

 winding rows, and exhibiting here and there the minute openings of the ordinary 

 zooidal tubes. The majority of specimens, however, exhibit additional features of 

 great interest and importance. In most specimens, namely, the surface everywhere 

 exhibits a considerable number of round apertures, which are placed at tolerably 

 regular intervals, are of much larger size than the openings of the ordinary zooidal 

 tubes, and are surrounded by thickened and elevated rims (Fig. 14, a). Longitu- 

 dinal and transverse sections show that these apertures are the openings of large 

 tubes, which are at first directed inwards, and then curve downwards as they 

 approach the axis of the stems. These tubes (Fig. 15, b b) are intersected by curved 

 or vesicular tabulae, and they appear to be provided near their mouths with thickened 

 proper walls. As they approach the axis of the stems, however, the thickened wall 

 disappears, and they seem to be only bounded by the general tissue of the skeleton ; 

 while they finally terminate by opening into the interlaminar spaces of the coenosteum. 

 Whether or not they have have any direct connection with the axial tube which 

 traverses each stem is a point very difficult to determine positively. The 

 phenomena presented by longitudinal sections would, however, seem to show that 

 such a connection certainly exists, in at any rate some instances. 



