Prof. W. King on Pleurodictyum problsmaticum. 131 



ticular is curious, from its resemblance to the North American 

 H. hirsuta, Say. 



Mr. Theobald's personal researches in the Punjab and Sikkim 

 have also added new forms from the Salt Range and Darjiliiig. 



Cheltenham, 12th January 1856. 



XII. — On Pleurodictyum problematicum. By William King, 



Professor of Mineralogy and Geology in Queen's College, 

 Galway, Corresponding Member of the Natural History and 

 Medical Society of Dresden, &c. 



[With a Plate.] 



A FEW weeks since I selected, from the extensive sale collection of 

 Dr. Krantz of Bonn, several fine specimens of the Pleurodictyum 

 problematicum of Goldfuss, from the Upper Devonian sandstone 

 of Daun in the Eifel. Previously, I had not examined any 

 examples of this singular fossil : all the information I then pos- 

 sessed respecting it was derived from some published figures and 

 descriptions by Goldfuss, Phillips, and Lyell ; and I had an im- 

 pression that the vermiform appendage, occurring within it, was 

 generally considered to be a foreign body. 



Pleurodictyum problematicum, as it usually occurs, may in 

 general terms be described as an oval or nearly circular discoid 

 body, having one surface free and the other firmly adhering to a 

 portion of the matrix in which it occurs. If observed attentively, 

 it will be seen to consist of a number of closely packed, more or 

 less inclined subpolygonal cones, with their apex or small end 

 corresponding to the free surface, and their base attached to the 

 matrix: the cones are at a slight distance from one another, 

 but connected by means of a number of short thread-like pro- 

 cesses crossing the vacant interspaces. Within the central area 

 of the free surface a sigmoid or S -shaped vermiform appendage 

 is seen lying among the interspaces, and having both termi- 

 nations passing down to the opposite or adhering surface. 



Sir Charles Lyell has given a tolerably correct view, natural 

 size, of the free surface of this fossil in his excellent Manual, 

 p. 429, 5th edit. The same surface is represented, twice the 

 natural size, in PI. X. fig. 1, so as to exhibit the difi^erent parts 

 more obviously. 



The fossil, as just described, is a cast ; it will therefore be evi- 

 dent, that the cones are casts of subpolygonal cells, — the vacant 

 interspaces, their walls, — and the short thread-like processes 

 crossing the same, casts of tubular openings or foramina in the 

 cell-walls. It will also be obvious, that the free surface exhibits 



9* 



