Lower Helderberg Fauna. 291 



marked extent the minutely porous structure which is so charac- 

 teristic of Stromatopora, properly so called; while the radial 

 pillars and their connecting processes are as distinctly and clearly 

 developed as in the type-forms of Actinostroma. The radial 

 pillars, in fact, are exceedingly large and give off whorls of deli- 

 cate ' arms ' or connecting processes which are emitted at cor- 

 responding levels in a radiating manner, and which circumscribe 

 rounded pores representing the zooidal tubes. The astrorhizal 

 canals are largely developed, and we therefor see in vertical sec- 

 tions, as in similar sections of S. densum, the large rounded aper- 

 tures which represent cut ends of these tubes, and upon which 

 the genus Syringostroma was originally based. This latter 

 character is, of course, one of no generic importance, as, indeed, 

 present in all Stromatoporoids with large astrorhizal canals." 



It may seem unjust to a writer to discuss his work without the 

 advantage of consulting identified specimens, but under the cir- 

 cumstances I have been forced to do so. After a careful study 

 of both text and plates, no other conclusion is possible than that 

 S. Mistigouchense is an Actikostroma. of no abnormal type, and 

 if the generic claims of Syringostroma rest on the validity of S. 

 Bistigouchense, it will have to be united with some other form. 



Nicholson well says that, in radial section, the structure is 

 characteristically Actinostromoid, but the statement that tan- 

 gential sections show the minutely porous structure of Stroma- 

 topora seems ill-advised. The skeleton of the latter genus is 

 porous, and the tissue itself minutely fibrous or vesiculose. 

 Kadial pillars are few or absent, and the porous skeletal tissue is 

 not formed by connecting processes springing from the arms, but 

 by independent anastomosing fibers. According to Nicholson, 

 however, this structure is represented in S. Eistigouchense^ and 

 is also typical of the genus Actinostroma. On this account I 

 have placed the species in that genus. The unusual appearance 

 in tangential section exhibited by Actinostroma Eisttgouchensa 

 seems to be due to the large size of the radial pillars, to which 

 Nicholson has called attention, accompanied by a corresponding 

 reduction in the length of the connecting processes. It is owing 

 to the same fact that, in vertical section, the laminated char- 

 acter of the skeleton is not as striking as in other species of 

 Actinostroma. 



