252 TABULATE CORALS. 



and there is generally but one of these structures present, 

 though two or three may be developed, in which case they 

 are very closely approximated. In the smaller corallites the 

 tabulse are more numerous (PI. XII., fig. 3 e), and they extend 

 to close upon the summit of the tubes. No traces, lastly, of 

 mural pores, or of any other apertures, in the well-developed 

 walls of either the larger or smaller corallites have hitherto 

 been detected. 



As regards the systematic position of Pinacopora, there can 

 be no doubt that it belongs to the family of the Helioporida:, 

 and that its nearest relationships are with Heliolites itself. It 

 agrees with Heliolites in the essential structure of the large 

 corallites, and also in the more important features exhibited by 

 the small tubes. The latter, it is true, in their comparatively 

 limited development, remind us of Plasinopora {Propora) tubu- 

 lata, E. and H., the regular distribution of the large corallites 

 being another feature of resemblance between the type just 

 mentioned and the present form. On the other hand, there is 

 the fundamental difference that the walls of the small corallites 

 in Pinacopora are completely differentiated, while their tabulse 

 do not become vesicular. In these features, Pinacopora agrees 

 with Heliolites, from which it differs in the fact that the cor- 

 allum constitutes a thin, concavo-convex, leaf-like plate, not 

 attached to foreign bodies ; in the extraordinary shortness of 

 the corallites ; in the comparatively rudimentary condition of 

 the septa ; in the very regular arrangement of the large coral- 

 lites ; and in the peculiar arrangement of the tabulse in the 

 large and small tubes respectively. As before remarked, the 

 only described species of this genus is found in deposits of the 

 age of the Upper Silurian, in Ayrshire. 



