PARALLELOPORA DARTINGTONENSIS. 201 



Barg., by its massive or laminar mode of growth, and the extensive development 

 and comparatively great size of the astrorhizas, as well as by the common 

 arrangement of these structures in vertical systems. It is, however, essentially by 

 the minute structure of the skeleton that the present species is separated from its 

 congeners, as well as from the species of Stromatopora and Stromatoporella. The 

 principal characteristic features in the internal structure are the distinctness of 

 the narrow radial pillars, and the regularity and slender form of the connecting 

 processes by which these are united. These characters give to vertical sections 

 (Plate XXV, figs. 1 and 3) the aspect of corresponding sections of an Actino- 

 stroma rather than of one of the Stromatoporidse proper. On the other hand, the 

 connecting processes of the pillars are not developed in whorls, as they are in the 

 species of Actinostroma ; and tangential sections show nothing, therefore, of the 

 " hexactinellid " structure so characteristic of the genus Actinostroma. On the 

 contrary, tangential sections (Plate XXIV, fig. 15, and Plate XXV, fig. 2) show 

 that the skeleton is, in the main, a reticulated one, as it is in the Stromatoporidae 

 generally. At the same time, owing to the distinctness of the radial pillars, and 

 also to the fact that their connecting processes are developed at regular levels, 

 the reticulation of the skeleton, as seen in tangential sections, is not so complete 

 as in the species of Stromatopora proper. Indeed, where the plane of such a 

 section happens to coincide with an interlaminar space (i. e. a space between two 

 contiguous sets of connecting processes) the reticulated character of the 

 skeleton is lost, and we see only the detached rounded or oval ends of the trans- 

 versely divided pillars. It was this phenomenon which induced me in the earlier 

 portion of this work to take the view that the present species might possibly find 

 a place in the genus Stromatoporella. Upon the whole, however, the characters 

 of the skeleton-fibre, so far as these have been ascertained, would show that the 

 species is properly referable to Parallelopora, with which genus it also agrees in 

 the possession of large " tabulate " astrorhizal tubes. 



The form with which the present species seems to be most nearly allied is 

 P. Goldfussii, Barg., and this alliance is particularly shown in a variety of the 

 species which I may term P. dartingtonensis, var. filitexta. This variety is dis- 

 tinguished from the normal form of the species by its more delicate skeleton- 

 fibre (Plate XXV, fig. 2), and by the less regular development of the radial 

 pillars (Plate XXV, fig. 3), tangential and vertical sections having thus a markedly 

 distinctive character communicated to them. In this variety also the peculiar 

 perforate character of the skeleton-fibre is more marked, or, at any rate, is more 

 conspicuous, than it is in the ordinary form of the species. 



Specimens of P. dartingtonensis often occur in a "reversed" condition, when 

 they are known to the Devonshire lapidaries as " stag-horn " specimens. In this 

 condition (Plate IV, fig. 1) the astrorhizal canals, as well as the other internal 



