216 PROF. H. A. NICHOLSON AND DR. J. MTJRIE ON THE 



with wide and open meshes. When examined with low powers, 

 and especially with the use of polarized light, the walls of the 

 vesicles sometimes exhibit the appearance of being traversed by 

 numerous vertical tubuli, placing contiguous vesicles in suc- 

 cessive layers in direct communication. This structure would be 

 very closely similar to that of the Foraminiferal genus Tinoporus ; 

 but we cannot assert positively that it exists, though we have 

 paid special attention to this point. The surfaces of the layers in 

 this form, when exposed in weathered specimens, usually show 

 nothing but the oval siliceous casts of the sarcode-chambers, 

 separated by vermiculate and anastomosing depressions. These 

 two forms appear to us to be so fundamentally distinct from 

 Stromatopora proper, in spite of their close general resemblance 

 to it, that we propose to give them a special name, viz. Clathro- 

 dictyon. We propose, further, to name the Clinton species G. 

 vesiculosiim, and the Corniferous form C. cellulosmi. 



(d) Lastly, we may note the occurrence of forms in most 

 respects resembling the Stromatoporoids in appearance and 

 general arrangement, but having merely a loosely reticulate and 

 indefinite minute structure. The only specimen we have of this 

 group is from the Cincinnati group (Lower Silurian) of Waynes- 

 ville, Ohio, and is completely silicified, the skeleton being con- 

 verted into flint, and the interspaces filled with transparent silica. 

 In general form the fossil is Stromatoporoid, composed of con- 

 centric shells, and having an undulating surface, elevated into 

 conical prominences. Internally the skeleton is irregularly reti- 

 culate and porous, traversed by canals which often radiate from 

 minor centres. Our investigations of this specimen, though 

 sufficient to indicate its distinctness as a separate type, have not 

 been carried so far as to justify us in giving it a separate title. 

 Forms of an apparently similar nature occur in the Corniferous 

 Limestone (Devonian) of Ohio, but we have not hitherto been 

 able to examine these minutely. 



10. Classification and Types of the Stromatoporoids. — The 

 Stromatoporoids have generally been grouped and classified in 

 accordance with certain well-marked external features, such as 

 the presence or absence of radiating horizontal water-canals, the 

 presence of vertical tubes penetrating the mass, &c. These 

 characters are sometimes sufficient (as in the case of the vertical 

 canals of Caunopora) ; but more minute examination has shown 

 that others, formerly relied upon as tests of generic distinction, 



