1886.] NATURAL SCIENCES OP PHILADELPHIA. 49 



spaces, therefore, appear as layers of rounded or elongated vesic- 

 ules somewhat resembling the tissue of Gystij^hyllum on a minute 

 scale. Forms of this structure occur abundantly in the Niagara 

 group and some in the Devonian strata. 



A generic distinction of these forms, represented in many dif- 

 ferent modifications, can, in my opinion, be made with propriety, 

 but as intermediate forms occur, in which the lamellae are only 

 little disturbed in their straight course by the inflexions repre- 

 senting the interstitial pillars, it is difficult to draw a line separ- 

 ating Clathrodictyon from Stromatopora. 



Prof. Nicholson's type form of Glathrodictyon^ occurring in 

 the Niagara group, is described as forming large cake-like expan- 

 sions from one to two inches in thickness centrall}^, but thinning 

 out near the margins. Upper face irregulai'ly undulating and 

 exfoliating concentrically round elevated points. Surface smooth. 

 Internal structure exceedingly delicate, formed of fine close-set, 

 horizontal or slightly undulating laminae, of which about twenty 

 to twenty -five occupy the space of one line. Interlaminar spaces 

 divided into minute lenticular cells formed by curved inflexions 

 of the horizontal laminae. 



Specimens to which, in a general way, this description is ap- 

 plicable, are common in the Niagara group of Michigan, Kentucky 

 and Iowa, but although a good many hundreds came under my 

 observation I could never find one whose structure was so minute 

 as to exhibit from twenty to twenty -five laminae on the space of 

 one line. The most delicately built among my specimens had not 

 over sixteen within the space of a line. 



The form having this delicate structure I had, in my above- 

 mentioned essa}- on Stromatopora, described and figured under 

 the name Stromatopora minuta. Its structure does not repre- 

 sent the character of Clathrodictyon in its most perfect develop- 

 ment, as the lamellae, which by their inflexion form the pillars, 

 are not much twisted out of their rectilinear general direction, and 

 as a number of pillars are observable in such sections which are 

 papillose prominences of the surface of the lamellae, not showing 

 any inflexion of the latter at their base. 



Associated with this form are several other forms of much 

 coarser structure with from six to eight lamellae within the space 

 of one line, which exhibit the clathrodictyon character in much 

 more ideal perfection than the type form does. These had been 



