GENERA OF CH^TETID.E AND MONTICULIPORID.E. 315 



M. petropolitana, in different deposits and in different countries, 

 have been based solely upon macroscopic investigation, and 

 that this is clearly insufficient for specific diagnosis, it has ap- 

 peared to me to be quite useless to give any synonymy of the 

 species. With our present knowledge, in fact, such a synonymy 

 would simply give us the information that certain authors had 

 identified from certain regions and formations corals which are 

 doubtless referable to MontiaiHpora^ in its wide sense, and 

 which resemble AI. petropolitana in form and habit. 



In the Swedish specimens of M. petropolitana, Pand., which 

 I shall take as the type of the species, the corallum has the 

 well-known hemispherical or sub-globular form, its circular and 

 concave base being covered with a thin concentrically striated 

 epitheca (PI. XIII., figs. 3, 3 a). In thin sections (PI. XIII., 

 figs. 3 b and 3 c) no feature is more striking than the ex- 

 treme delicacy and tenuity of the walls of all the coral- 

 lites. 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 Monticuli- 

 pora {Heterotrypa). In tangential sections (PI. XIII., fig. 3 b) 

 another marked peculiarity is the strictly angular form of all 

 the tubes, and the very regularly hexagonal or pentagonal 

 outline of the larger 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 (PI. XIII., fig. 3 c) the two 

 sets of corallites are chiefly recognisable by the difference in 

 their respective sizes, their tabulation being more uniform 

 than is usual in the genus. The small corallites are, how^ever, 

 always to some extent more closely tabulate than is the case 

 with the large tubes. 



