200 BRITISH STROMATOPOROIDS. 



The coenosteum in P. dartingtonensis, Cart, sp., is massive or laminar in form, 

 and often attains a considerable size ; the mode of attachment and condition of the 

 under surface is not thoroughly known. The mode of growth is often more 

 or less clearly latilaminar, though in many specimens this feature is not con- 

 spicuous. The concentric lamina? are generally simply curved or slightly 

 undulated. The astrorhiza? (Plate XXIV, figs. 13 and 14) are extensively but 

 variably developed, consisting usually of large stellate branching canals, the 

 cavities of which are crossed by more or fewer internal partitions (" astrorhizal 

 tabula?")- Commonly the astrorhiza? are arranged in vertically superimposed 

 systems, each system being connected with an axial wall-less canal ; but this dis- 

 position of parts is not always recognisable. Owing to the curvature of the 

 concentric lamina?, the astrorhizal canals commonly exhibit a more or less 

 truncated or irregular disposition in straight tangential sections (Plate XXIV, 

 fig. 13), while their cut ends appear at intervals in vertical sections as large 

 rounded or oval apertures. The centres of the astrorhiza? are usually from 

 1 to 2 cm. apart. The condition of the surface is imperfectly known, but 

 it is probable that there were more or less pronounced eminences or "mamelons," 

 corresponding with the centres of the astrorhizal systems. 



As regards the minute structure of the skeleton, an examination of the surface 

 of polished specimens with a pocket lens clearly shows the skeleton-fibre to 

 possess the perforate or cribriform character which distinguishes the species of 

 Parallelopora. Owing, however, to the bad state of preservation of most 

 specimens, this feature is often more or less completely lost in thin sections ; 

 though indications of it may be observed in most examples, and in a few cases the 

 structure in question can be clearly observed (Plate XXV, fig. 2). 



When examined in thin tangential sections (Plate XXIV, fig. 15; and Plate 

 XXV, fig. 2) the coenosteal tissue is seen to be essentially of the reticulate type, 

 though the reticulation is not so complete as in the species of Stromatopora proper. 

 Such sections also commonly show the transversely partitioned astrorhizal canals. 

 Vertical sections (Plate XXV, figs. 1 and 3) show the coenosteum to be made up 

 of delicate, close-set, and very distinct radial pillars, which are united at short and 

 regular intervals by comparatively thin horizontal cross-bars, these latter giving 

 rise to the appearance of regular and closely arranged " concentric lamina?." 

 About nine or ten pillars occupy a space of 2 mm. measured transversely, and 

 about the same number of " concentric lamina? " occupy this space measured 

 vertically. The spaces between the radial pillars represent the zooidal tubes, and 

 these, in well-preserved specimens, exhibit a moderate number of straight or 

 curved tabula?. 



Obs. — As regards its larger characters, so far as these are known, Paral- 

 lelopora dartingtonensis is chiefly distinguished from the allied P. Goldfussii, 



