GENERA OF CH.ETETIDM AND MONTICULIPORIDyE. 313 



Monticulipora (Diplotrypa) petropolitana, Pander. 

 (PI. XIII., figs. 3-3^.) 



Favosites pdrofolitanus. Pander, Russ. Reiche, p. 105, PI. I., figs. 6, 7, 10, 11, 



1830. 

 (Non ChcBietes petropolitamis, Nicholson, Quart. Journ. Geol. Soc, vol. xxx. 



p. 510, PL XXX., figs. 5-8, 1874; Geol. Mag. Dec. ii., vol. ii. p. 175, 



1875; P^l- Ohio, vol. ii. p. 204, PL XXL, figs. 14-14 b, 1875; Ann. 



Nat. Hist., ser. 4, voL xviiL p. 88, PL V., figs. 6-6 «.) ^ 



Spec. Char. — Corallum discoid when young, but spheroidal, 

 or hemispherical when fully grown, the base being circular, 

 more or less deeply concave, and covered with a concentrically 

 striated epitheca, while the calices cover the whole of the upper 

 surface. The corallites are of two sizes, large and small, these 

 being uniformly interspersed with one another throughout the 

 entire colony, while the former also constitute small clusters or 

 monticules. The large corallites are about one-quarter of a 

 line in diameter, provided with uniformly thin and delicate 

 walls, not thickened towards the surface, and for the most part 

 very regularly hexagonal in shape. The small corallites are 

 wedged in at the angles of junction of the large tubes, which 

 they sometimes to a large extent separate from one another, 

 their diameter varying from a twelfth to an eighth of a line or 

 more. They resemble the large corallites in being uniformly 

 thin- walled and strictly angular, their shape being very variable, 

 but mostly oblong, square, or sub-triangular. Both sets of 

 tubes are provided with complete horizontal tabulae, which 

 increase in number towards the surface ; and the tabulae in the 

 smaller tubes are more numerous than in the larger ones, 

 though this disproportion is not so marked as is usually the 

 case in the species of Monticulipora. 



Obs. — A great number of corals have been described or 

 quoted by different authors from the Lower Silurian deposits of 



1 Beyond pointing out that the forms which I have previously considered and 

 described as M. petropolitana, Pand., are really distinct from the original Russian 

 type of the species, I have not thought it — for reasons to be subsequently given — of 

 any use to attempt to give a synonymy of this form. 



