S UB- GENUS DIPL TR YPA . 



159 



concave base being covered with a thin concentrically striated 

 epitheca (fig. 30, a and b). In thin sections no feature is more 



30. — JMottticjilipora petropolitana, Pander. A, A specimen viewed in piofile, of tlie 

 natural size ; B, Under view of the same, showing the basal epitheca, of the natural 

 size ; c, Part of a tangential section of tlie same, enlarged twenty times ; D, Part 

 of a vertical section of the same, enlarged twenty times. From tlie Lower Silurian rocks 

 of Sweden. 



striking than the extreme delicacy and tenuity of the walls of 

 all the corallites. The walls are so thin that they appear as 

 mere simple and undivided dark lines, the originally duplex 

 character of the boundaries between contiguous corallites 

 being entirely lost. Nor, again, do the walls become in any 

 way thickened as the surface is approached. In this respect, 

 therefore, there is a marked and important difference in the 

 structure of this form as compared with the more normal types 

 of Monticulipora {Heterotrypa). In tangential sections (fig. 

 30, c) another marked peculiarity is the strictly angular form 

 of all the tubes, and the very regularly hexagonal or pentag- 

 onal outline of the larorer corallites. Each of the larger tubes 

 is usually in contact with a tube of the same series on one or 

 two sides, but the other faces usually abut against corallites of 

 the smaller series, these being generally oblong or quadrate in 

 shape. In vertical sections (fig. 30, d) the two sets of corallites 

 are chiefly recognisable by the difference in their respective 



