SUB-GENUS DIFLOTRYPA. 157 



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 i-50th inch 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 some- 

 times to a large extent separate from one another, their diam- 

 eter varying from i-5oth or less to i-iooth inch. They resem- 

 ble 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 pro- 

 vided with complete horizontal tabulze, which increase in num- 

 ber towards the surface ; and the tabular in the smaller tubes 

 are more numerous than in the larger ones, though this dispro- 

 portion 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 

 different parts of the world under the name of Monticulipora 

 petropolitana or Chcetctcs petropolitamts, Pand. In most cases, 

 however, the determination of particular specimens as belong- 

 ing to this species has been founded upon the well-marked 

 external form of the corallum and the general nature, often 

 with difficulty recognisable, of its sw-'face-c/iaractcrs. That 

 this should be the case was inevitable, seeing that the internal 

 structure of the corallum does not admit of being made out 

 properly save by means of carefully prepared sections ; and 

 further, that the minute characters of the genuine Russian types 

 have only been investigated and published within quite recent 

 times (Dybowski, Die Chcetetiden der Ostbaltischen Silur- 

 formation, 1877), ^^ the time when I published my work 

 upon the ' Structure and Affinities of the Palceozoic Tabulate 



