150 Dr. Alex. Broicn — On Solenopora. 



found in the species to be next described, viz. Solenopora Jurassica, 

 Nicholson. 



Formation and Locality. — Ordovician Limestones, Saak, Esthonia, 

 Eussia. (ColL H. A. Nicholson.) Here it occurs along with 

 S. compacta and S. dendriformis. 



4. Solenopora Jurassica, Nicholson (MS.). 



This form occurs in subspherical masses varying from the size of 

 a marble to that of an orange or even larger. The fractured surface 

 has a porcellanous appearance, and shows the arrangement in con- 

 centric layers very markedly. Each layer has a reddish-brown 

 colour internally, fading into a whitish appearance externally. 



In vertical section the cells are arranged in a radiating fashion. 

 They have about the same diameter as those of Solenopora compacta 

 (about ^Vmm.), and their length varies from |mm. to ^ mm. The 

 arrangement of the cells is upon the same principle as in Solenopora 

 nigra, and they do not group themselves into layers corresponding 

 to the concentric strata seen with the naked eye. The cell-walls 

 are very distinct, and not sinuous. The transverse walls are slightly 

 concave on the peripheral side (Fig. 4). Certain zones are observed 



Fig. 4. — Long. sect, of Solenopora Jurassica, Nich. (MS.), from Chedworth, 

 Gloucestershire. X 50 d. 



to have their walls either very indistinct or obliterated by crystal- 

 lization. These zones correspond to the more lightly coloured 

 portions of the concentric layers of the mass as visible to the 

 unaided eye. Some sections show oval or rounded spaces here and 



