142 TABULATE CORALS. 



known with certainty from the Devonian and Carboniferous 

 rocks ; since the forms which have been noted as occurring in 

 the Upper Silurian deposits of North America appear to be 

 small discoid examples of the type of Pkiirodictyum. My own 

 collection includes excellently-preserved specimens of M. con- 

 vexa, D'Orb., M. favosoidea, Bill., and M. {Emmonsici) cylhi- 

 drica, E. and H. ; but as these are not only silicified, but have 

 their tubes entirely empty, they do not admit of being satisfac- 

 torily sectioned for the microscope. Through the kindness of 

 Mr Etheridge, I have also had the opportunity of examining 

 specimens of M. megastoma, PhilL, RI. temdscpta, Phill., and 

 M. favosa, Goldf , from the Museum of Practical Geology. 

 As, however, I am not in the position of being able to give any 

 details as to the microscopic structure of any of the above, it 

 will be unnecessary to select any species for description. 



Genus Pleurodictyum, Goldfuss, 1829. 

 (Petref. Germ., t. i. p. 113.) 



Gen. Char. — Corallum discoidal, with a slightly convex 

 upper surface, attached to foreign bodies by the centre of its 

 base, and having its lower surface covered by a well-developed, 

 concentrically-striated epitheca. Corallites diverging from the 

 centre of the base, those on the circumference being nearly 

 horizontal, while the median ones are more or less nearly per- 

 pendicular ; their general shape polygonal or subcylindrical. 

 Walls of considerable thickness, pierced from side to side by 

 mural pores, which have no regular arrangement. Tabulae not 

 very numerous, nor markedly arched ; often inosculating to 

 some extent, but not giving rise to a vesicular or subvesicular 

 tissue. Septa rudimentary, in the form of marginal ridges or 

 rows of vertically-disposed spines. 



Obs. — This genus was founded for the reception of the 

 curious P. problematieum of the Devonian deposits, which is 

 still only known by means of casts of the tubes in sandstone. 



