38 PROF. E. W. CLAYPOLE ON HELICOPORA. 



as an occasional " sport," or lusus waturce. Possibly such forms may 

 yet be found in imperfectly known districts ; but it is almost idle to 

 expect them from rocks so well searched and so long studied as the 

 Mountain Limestone of England. Meanwhile these beautiful fossils 

 form an exceedingly interesting example of local development, and 

 supply an argument of some weight in favour of the separation of 

 the inland basin of North America from that of Europe, either by 

 physical barriers or impassable conditions during the period of their 

 rise, culmination, and decay. 



Note. Sejrtember 18, 1882. — Since the above paper was written, 

 Prof. James Hall has described in a small pamphlet sixty-three species 

 of Fenestella and Polypora from the Upper Helderberg and Hamilton 

 groups of the Devonian in the United States. Of these sixty-three 

 species, forty are infundibuliform, one cup-shaped, one flabellate, and 

 twenty-one indeterminate. Not one of them is spiral. 



EXPLANATION OP PLATE IT. 



Fig. 1. Helicopora latispiralis, lower face, showing four whorls of the spiral, 

 nat. size. a. A small portion enlarged 2 diameters. 



2. Ulrichi, enlarged 2 diameters, a. A small portion enlarged 



8 diameters. 



3. archimediformis, nat. size, 



4. Enlarged view of a poriferous part of fig. 3, magnified 4 diameters. 



