170 BRITISH STROMATOPOROIDS. 



zooidal tubes, with their intervening radial pillars, occupy a space of 2 mm. 

 measured at right angles to their length. Tangential sections (Plate XXII, fig. 1) 

 show the finely reticulated skeletal network, pierced by the generally round 

 openings of the transversely divided zooidal tubes and traversed by the branching 

 astrorhizal canals. The latilaminar structure of the skeleton is also well 

 exhibited by vertical sections. 



Obs. — The form of the coenosteum in 8. typica is essentially discoidal, 

 with a basal epitheca, the smallest example seen being 1^ cm. in diameter. 

 Young specimens (Plate XXI, figs. 4—6) are thin, approximately circular discs, 

 fixed basally to foreign objects by a small peduncle of attachment, or, at other 

 times, by a large portion of the under surface. In some cases the discoidal or 

 laminar form is more or less completely retained throughout life, few latilaminae 

 being produced, and these being widely extended laterally. More usually, the 

 successively produced latilaminae not only extend beyond the margins of the 

 previously formed disc, the coenosteum thus increasing in diameter ; but each 

 stratum is thicker in the middle than at the periphery, so that the colony assumes 

 a hemispherical shape, with a flat or concave base (Plate XXI, fig. 8). Large 

 specimens may exceed a foot in diameter, but the hemispherical form is usually 

 more or less closely retained. 



Latilaminar growth is almost as marked a feature as in 8. concentrica, Goldf., 

 each latilamina consisting of a single layer of zooidal tubes. The latilaminae are 

 always in gentle curves or slight undulations, conforming with the surface of the 

 hemispherical coenosteum. The astrorhizae of 8. typica are characteristic in their 

 great numbers, small size, and few straggling branches (Plate XXI, fig. 7). Usually 

 their centres are 5 or 6 mm. apart, but they may be more widely spaced than this. 

 An arrangement of the astrorhizae into vertically superimposed systems, each 

 with a common axial canal, can often be made out ; but this is not a conspicuous 

 feature, and "astrorhizal cylinders" are never developed, the surface of each 

 successive latilamina being thus devoid of eminences or " mamelons " corre- 

 sponding with the astrorhizal centres. 



The skeleton-fibre (Plate I, fig. 3, and Plate XXI, figs. 9 and 10) is minutely 

 porous, and this structure is more or less clearly recognisable in all well-preserved 

 examples. In some examples, however, the skeleton-fibre appeal's to have under- 

 gone a sort of change, in virtue of which it appears in vertical sections as if 

 traversed by innumerable perpendicular and horizontal dark striae. This 

 appearance has been figured by Baron von Rosen ('Ueber die Nat. der Strom.,' 

 Taf. i, fig. 2), and is not uncommonly seen in specimens from Gotland or Bsthonia, 

 but only in examples which can be otherwise shown to have undergone more or 

 less alteration. As has been previously pointed out (p. 145), vertical sections of 

 specimens in which the skeleton-fibre has been altered in the way just described 



