Dr. Alex. Brown — On Solenopora. 149 



the cells. These spaces are well displayed in specimens from the 

 Trenton limestones and from Esthonia, which specimens are 

 assuredly examples of Solenopo7-a compncta.^ In the specimens of 

 S. compacta hitherto described no definite proof has been found 

 that these spaces do correspond to cell- walls. In Solenopora litho- 

 thamnioides, however, distinct walls are seen which agree in position 

 with the clear markings seen also in the same species (see PI. V. 

 Fig. 2). 



Formation and Locality. — Ordovician (Silurian?), Shalloch Mill, 

 near Girvan, Ayrshire. (Coll. H. A. Nicholson.) 



3. Solenopora nigra, n.sp. (PI. V. Fig. 3.) 



This species presents itself in small oval, ovate, or rounded masses 

 up to the size of a pea. It has always a dark appearance, and it is 

 very difficult to prepare good transparent sections from it. When 

 fractured the concentric arrangement in layers is well seen, and 

 the surface presents a porcellanous or somewhat fibrous aspect. In 

 vertical section the distinct concentric layers of cells peculiar to 

 the last species are not nearly so manifest. The cell-walls are 

 thick, dense, and feebly sinuous, the transverse equalling the 

 longitudinal in thickness. Cells -gV — -so mm. wide and about 

 i — T^7- mm. long. The cells near the point from which radiation 

 proceeds are much narrower and longer than the peripheral cells. 

 The whole cellular tissue often tends to assume a fan-like appearance. 

 In tangential section the cells appear rounded or oval, and some- 

 what sinuous in their outline. Walls thick, and cell-division active 

 (Fig. 3). 



Fig. 3. — Tang. sect. Solenopora nigra, n.sp., from Esttonia. X 50 d. 



Observations. — This species occurs in great masses of limestone, 

 of which it is one of the principal constituents, and such is the 

 appearance of a mass of this limestone that one might readily 

 mistake it for an ordinary Nullipore limestone such as we meet 

 with in the Tertiary series. One is struck, also, with the similarity 

 in form, appearance, and arrangement of the cells, to what is 



^ These concentrically arranged clear spaces were alluded to by Prof. Nicholson in 

 the Geol. Mag. Dec. III. Vol. II. No. 12, p. 533, December, 1885. He showed 

 there also that, corresponding to the clear spaces in the tubes, there were inter- 

 ruptions in the longitudinal walls. 



