Maryland Geological Survey 223 



fused, but not sufficiently so to obliterate their individuality. On an 

 average seven pillars appear in a distance of 1 mm. The skeletal matter is 

 rather coarsely porous. 



'• Vertical sections show the vertical ajul horizontal elements forming 

 a network with relatively thick meshes. The cut ends of the astrorhizal 

 canals are apparent as interspaces of a more or less rounded character, and 

 the zooidal pores as minute tabulate tubes between the pillars. When the 

 section is thick the lamina' are most pronounced and continuous, because 

 the zooidal tubes are of such small caliber that the section exceeds them 

 in thickness; the thinner the section the more zooidal tubes are seen to 

 hold their course through the laminsp. This is an obvious fact, but one 

 must remember it. or tlieic is gra\e danger of misinterpretation. 



"Tangential sections show the astrorhizal canals as prominent features, 

 and the minute round orifices representing the cross-section of the zooidal 

 pores." 



The individuals referred to this species in Maryland may be grouped 

 into three rather well-defined types as follows : 



Type a. Characterized by possessing a coenosteum of medium size, 

 laminaj strongly curved, so that they appear very curly when weathered, 

 mamelons small or absent, atrorhizfe inconspicuous, their branches short, 

 colonies forming thick reefs of great extent. This is the most abundant 

 type and agrees well with Parks' description. 



Type b. Differs from preceding chiefly in much largx'r size of eo'nosteum 

 which is often more than 12 inches in diameter, lamina' less curved, 

 mamelons often more conspicuous, colonies less given to forming reefs. 



Type c. Possessing large mamelons and very large intricately branched 

 astrorhiza; which may even be nearly a centimeter in diameter. The 

 astrorhiza^ resemble those of Parks' iS'. comllifera so that tangential sec- 

 tions strongly suggest that species. It differs from it in its thicker laminae 

 and more distinct latilaminje, in the size of the mamelons and the 

 astrorliiza\ It corresponds with Hall's figures of *S'. constcUatd with 

 Mdiich it also accords in its intimate structure. 



This is the most abuiKhmt stromatoporoid of the Helderberg and forms 

 extensive reefs in tlie upper part of tlie Keyser member. 



