292 Prof. H. A. Nicholson — On a New Silurian Coral. 



The structure of Desmidopora alveolaris can be readity made out 

 by means of thin sections, if these happen to be taken from a 

 specimen in which the skeleton has not been much altered by mine- 

 ralization. In tangential sections of such specimens (PL VIII. Fig. 4), 

 the walls of the corallites are seen to be of moderate thickness, and 

 to exhibit no traces of the primordial wall. No traces of septal 

 structures are to be detected, and the visceral chambers of many of 

 the corallites are directly continuous in winding series. In vertical 

 sections (PL VIII. Figs. 5 and 6), the mural pores are seen at the 

 points where the plane of the section happens to coincide with the 

 walls of the corallites, and their presence is further recognized by 

 the common occurrence of interruptions to the complete continuity 

 of the walls. Where the section happens to intersect one of the 

 rows of confluent corallites in any direction except a directly trans- 

 verse one, the tabula? are seen to be vesicular, but at other points 

 they are as a rule complete. 



In other specimens the skeleton has undergone considerable 

 secondary change, and the appearances presented in these cases are to 

 a large extent different to. what they are in normal examples of the 

 species. Thus, in such cases (PL VIII. Figs. 7 and 8), the walls of the 

 tubes are considerably thickened, and in tangential sections exhibit 

 a peculiar beaded appearance. The same thickening of parts of the 

 walls of the corallites is shown in vertical sections. The first sec- 

 tions which I examined were taken from a specimen which had been 

 mineralized in this peculiar way : and I was led from the phenomena 

 which they presented to regard the species as being a peculiar form 

 of Labechia, which I named L. alveolaris, without giving any fignre 

 or description of it (Mon. Brit. Stromatoporoids, p. 83). Such sec- 

 tions do, in fact, show a curious likeness to Labechia, and particularly 

 to certain specimens of L. conferta. It need hardly be said, however, 

 that an examination of sections of a well-preserved example of 

 Desmidopora alveolaris leaves us in no doubt as to the systematic 

 position of the species. 



Formation and Locality. — Eare in the Wenlock Limestone of 

 Dudley. I am indebted for all my specimens to the kindness of 

 Mr. William Madeley, of Dudley. 



EXPLANATION OF PLATE VIII. 



Fig. 1. Under side of a broken specimen of Desmidopora alveolaris, Nich., of the 



natural size, showing the epitheca. 

 ,, 2. Part of the surface of the same, enlarged six times. In the portion 



figured most of the calices are confluent. 

 ,, 3. Another part of the surface of the same, enlarged six times. In this 



portion of the surface a good many of the calices are definitely circum- 

 scribed. 

 ,, 4. Tangential section, enlarged six times. The specimen from which this was 



taken is but slightly altered by mineralization. 

 ,, 5. Vertical section of the same specimen, enlarged six times. 

 ,, 6. Part of the preceding section, enlarged twelve times, showing mural pores. 

 ,, 7. Tangential section of a mineralized specimen, enlarged twelve times. The 



appearances presented in this section are not at all unlike those 



exhibited by some forms of Labechia. 

 „ 8. Vertical section of the preceding specimen, enlarged twelve times. Here, 



again, the appearances are very like those shown by vertical sections of 



Labechia conferta, Lonsd. 



