13G BRITISH STROMATOPOROIDS. 



cosum, as regards its minute structure, is not separable from the normal form of 

 A. clathratum. Specimens of the former are, however, readily distinguished from 

 those of the latter by the prominent " mamelons " on the surface, while weathered 

 or fractured surfaces at once reveal the presence of astrorhizal cylinders. The 

 general features of A. verrucosum are singularly constant ; but some specimens 

 have comparatively small " mamelons." 



Distribution. — A. verrucosum appears to be exclusively confined to the horizon 

 of the Middle Devonian. In the Devonian Limestones of Devonshire it is a 

 decidedly rare species, and I am only acquainted with it as occurring in the pebbles 

 from the Triassic conglomerates of Teignmouth. In the Middle Devonian of 

 Germany the species is comparatively abundant, occurring commonly in the 

 Paffrath area (Bi'ichel, Bosbach, &c), and being less frequent at Gerolstein and 

 Sotenich. 



3. ACTINOSTEOMA BIFARIUM, Nidi. PL XIII, figs. 3 — 7. 



Actinostkoma bifaeium, Nich. Ann. Nat. Hist., ser. 5, vol. xvi, p. 231, pi. iv, 

 figs. -1 and 5, 1SSC. 



The coenosteum in this species is massive, spheroidal or hemispherical in shape, 

 and of considerable size. The base has not been observed, and the surface is also 

 unknown. Irregular astrorhizas are sometimes present, but they are not exten- 

 sively developed and are sometimes not to be recognised at all. 



As regards internal structure the radial pillars are of two sizes, large and small. 

 The large pillars are very stout, and are much fewer in number than the small 

 pillars, their development being irregular, so that they may be from 5 mm. or less 

 to more than 1 mm. apart. The small pillars are very numerous, and are set more 

 closely together, and both sets are connected by numerous radiating " arms " 

 (Plate XIII, fig. G), which enclose angular zooidal pores. The concentric lamina? 

 are well developed, and there are from four to six interlaminar spaces in the space 

 of one millimetre. 



This well-marked species is distinguished at once from all the other recorded 

 forms of Actinostroma by the possession of a series of specially large radial pillars 

 interspersed among the ordinary pillars of the coenosteum. As growth does not 

 take place by well-marked " latilamina)," the pillars are apparently continued for 

 indefinite distances, and their development is always more or less irregular. The 

 shape of the zooidal pores depends upon the state of preservation of the specimen, 

 or is really liable to variation in different individuals. Thus the zooidal pores are 

 usually distinctly angular, but in some cases the " arms " connecting the pillars 



