imp erfectly -known Species of Stromatoporoids. 7 



however, prolonged downwards * into the interlaminar space 

 below in the form of a more or less complete radial pillar. 

 Some of the radial pillars are quite short ; others project 

 about halfway into the interlaminar space ; others cross the 

 space and connect themselves with the lamina below ; finally 

 a few spring from the upper sides of the larainEe. A further 

 A'erj characteristic point about the radial pillars is that they 

 are very commonly double at their bases, where they spring 

 from ^their producing lamina. Tangential sections (PI. I. 

 fig. 10) of this species are much more characteristic than is 

 usual in the genus Clathrodictyon. Where such a section 

 traverses an interlaminar space the cut ends of the radial pillars 

 are seen in the form of dark granular masses of considerable 

 size and usually of a more or less elongated or oval shape. 

 When the section more or less closely coincides with a con- 

 centric lamina, the cut ends of the radial pillars are more 

 closely set and larger in size, and often form a sort of mosaic 

 pavement, or at other times a loose reticulation. Tangential 

 sections are also unlike similar sections of most species of this 

 genus in the apparent absence of astrorhizal canals. 



Ohs. I shall have tlie opportunity of dealing more fully 

 elsewhere with the peculiarities of the minute structure of 

 C. striatellum^ and need not discuss them further here. In its 

 general features the species is one which can hardly be con- 

 founded with any other member of the genus. It is most 

 nearly related to G. regulare, Rosen, but is readily distin- 

 guished by its wider interlaminar spaces and by the quite 

 peculiar form of its radial pillars. I need only add that my 

 identification of this form as being the one which d'Orbigny 

 had in view in establishing his species is based upon an 

 examination of Lonsdale's original specimen, which served as 

 the type of the species to the French palaeontologist. My 

 identification of Stromatopora mammillata, Fr. Schmidt, with 

 d'Orbigny's species is based upon specimens of the former 

 kindly given to me by Magister Schmidt himself; and there 

 can be no doubt as to the complete identity of the two types, 

 such slight differences as are apparent being the result of the 

 fact that the Esthonian specimens are silicitied. 



Formation and Locality. Ordovician and Silurian. So far, 

 the species has not been recognized in the Ordovician rocks 



* lu the illustrations wlaich I gave of vertical sections of this species in 

 the introduction to the ' Monograph of British Stromatoporoids ' (pi. i. 

 fig. 1 and pi. V. fig. 3) the figures were inadvertently reversed in position, 

 80 that the radial pillars are represented as growing from ihQupper sides of 

 the laminse instead of from the lower, as is really the case. 



