THECID^ AND HELIOPORID.-E. 251 



graph of the Silurian Fossils of Girvan (Fasc. I.), by Mr R. 

 Etheridge, jun., and myself, in which the only known species of 

 the genus — viz., P. Grayi, of the Upper Silurian rocks of Ayr- 

 shire — is fully described. As I have no further material of this 

 species in my hands, I shall content myself on the present occa- 

 sion with the following brief remarks, most of which are taken 

 from the work just alluded to. 



The corallum in Pinacopora is free, and has the form of a 

 thin circular expansion, the under surface of which is covered 

 with a concentrically-striated epitheca (PI. XII., fig. 3). The 

 epitheca is always symmetrically concave, and its upper convex 

 surface (PI. XII., fig. 3 a) carries the corallites, which are re- 

 markable for their extreme shortness, when we take into 

 account their comparatively large size. Thus in a specimen 

 one inch in diameter the height of the corallites is only about 

 half a line, while the diameter of the larger tubes is equal to 

 their height. The corallites are divided into two groups, the 

 relations of which to each other are remarkably uniform. The 

 larger corallites are arranged upon the upper surface of the 

 epitheca with great regularity in obliquely intersecting rows, 

 each individual tube being isolated and separated from its 

 fellows by a zone of the smaller tubes (PI. XII., fig. 3 b). 

 Usually but one single row of the small corallites intervenes 

 between any given pair of the larger tubes, but occasionally 

 and here and there a few extra interstitial tubes may be 

 developed. The large corallites, except in their extreme 

 shortness, are constructed upon the type of the correspond- 

 ino- tubes in Heliolites and its allies, the resemblance between 

 the two being especially manifest in the presence in each of 

 twelve short septa (PI. XII., fig. 3 d). As in Heliolites, the 

 smaller corallites are wholly destitute of septa. Considering 

 the very limited vertical development of the corallites, the 

 tabula; may be said to be well developed, and they differ 

 in their arrangement in the large and small tubes respec- 

 tively. In the former, the tabula; (PL XII., figs. 3 c and 

 3 e) are confined to the bottom of the visceral chamber, 



