Dr. Alex. Brown — On Solenopora. 197 



7. Solenopora fusiformis, n.sp. (PL V. Fig. 4.) 



This species occurs as small white nodules, oval, ovate, or spherical 

 in form. Concentric rings faintly seen. Its size varies from the 

 dimensions of a pea to those of a hazel-nut. The structure is 

 generally greatly obscured by crystallization. 



In longitudinal section numerous concentric rings are seen, but 

 many of them are altered by secondary crystallization. The cells 

 have about the same diameter as the cells of Solenopora filiformis, 

 viz. aVmm., but are more or less spindle-shaped, and dove-tail into 

 one another. 



In tangential section the outline of the cells is only feebly sinuous. 

 The cells vary in size considerably on account of their fusiform 

 nature. 



Observations. — The interesting feature here is the fusiform nature 

 of the cells, which can be more readily distinguished under a high 

 power. In this type, the concentric rings are particularly well 

 developed, and the general structure reminds one of such a form as 

 Litliothamnion. 



Formation and Locality. — Ordovician, Craighead Limestones, 

 Girvan, Ayrshire, occurring in great quantity, and forming large 

 masses of limestone. It is associated with S. compacta and 

 S. filiformis. (Coll. H. A. Nicholson.) 



II. Kelations of the Genus Solenopora. 



The observations previously recorded render it possible to arrive 

 now at something like a definite conclusion as to the affinities and 

 systematic position of the genus Solenopora. 



In the Tertiary rocks, and among existing Alg», we find the well 

 known Nullipores, many of which are massive in structure. Their 

 distribution is very wide, recent forms occurring as far north as 

 the Arctic region. They form extensive masses of limestone, as the 

 Tertiary era bears witness. To this group of Coralline Algae the 

 forms above described, as helping to give rise to thick masses of 

 limestone during the Ordovician and Jurassic periods, appear to 

 be closely related. 



FiG. 8. — Long. sect, of Litliothamnion facundnm. x 40 d. (after Kjenman). 



In Fig. 8 and PI. V. Fig. 1, the general structure of a Nullipore 

 {Lithothamnion) is seen. It consists simply of a mass of cellular 

 tissue arranged in concentric rings, which rings are often irregular. 

 The cells are, in section, quadrilateral in shape, and are of very 



