40 BRITISH STROMATOPOROIDS. 



lamina? and interlaminar spaces. On the other hand, in the genus Stromatopora, 

 Goldf., itself, and in some other forms, the interlaminar spaces become reduced to 

 rows of irregular chamberlets, or may even be almost obsolete (Plate V, fig. 15, 

 and Plate VII, fig. 2). 



Growth of the skeleton in the Stromatoporoids is effected by the upward exten- 

 sion of the radial pillars, and the production of successive concentric lamina? from 

 their apices. In many Stromatoporoids there occurred, in addition, periodic pauses 

 in the upward growth of the pillars and in the production of new lamina?, giving 

 rise to a sort of major stratification of the skeleton. That is to say, the skeleton 

 is now not only composed of successive " concentric laminae," but these in turn 

 are arranged in concentric strata of comparatively considerable thickness. 

 Successive strata may be in contact, or may be separated by incomplete inteiwals, 

 which are sometimes partially filled up with the matrix (Plate V, figs. 8 and 9). 

 In any case, the fossil splits more easily along the lines of division between 

 successive strata than elsewhere. I shall apply the term of " latilamina? " to these 

 thick strata, which result from an intermittent method of growth. They are very 

 conspicuous in some types of Actinostroma (e. g. A. clathratum, Nich.) ; but they 

 are exhibited in perfection in many species of the genus Stromatopora, Goldf., 

 and particularly in the type-species 8. concentrica, Goldf. (Plate XI, fig. 15). 

 There, is, moreover, this difference between the " latilamina? " in the cases just 

 mentioned. In Actinostroma, namely, each " latilamina " is made up of a series of 

 subordinate " concentric lamina? ;" whereas in the true Stromatoporce the proper 

 " concentric lamina? " can not be said to have any recognisable existence, or are, at 

 any rate, imperfectly developed ; so that the " latilamina? " have no tendency to 

 split along a subordinate series of concentric layers. 



As regards the general arrangement of the "radial pillars" and "concentric 

 lamina?," the genera Stromatopora, Goldf., and Actinostroma, Nich., may be taken 

 respectively as types of the two principal sections of the Stromatoporoids, namely, 

 the Milleporoid and the Hydractinioid sections. In the genus Stromatopora, Goldf., 

 the radial pillars and concentric lamina? are completely amalgamated with one 

 another, and are hardly recognisable, as a rule, as distinct structures. Hence, in 

 tangential sections of such forms (Plate V, figs. 14 and 15, and Plate XI, fig. 16) 

 the skeleton is seen to be a continuous reticulation, resembling that of MiUepora. 

 In vertical sections of the same, the radial pillars can usually be recognised to be 

 present, but they are thick, irregular, and flexuous, and the " concentric lannna? " 

 are only represented by irregular lateral outgrowths, which spring from the pillars 

 and unite them into a continuous framework (Plate V, figs. 15 and 17, and Plate 

 XI, fig. 18). In certain of the Stromatoporce, however, though the skeleton has 

 the completely reticulate structure which characterises the genus, the " radial 

 pillars," nevertheless, persist as distinct structures. Thus, in tangential sections 



