314 TABULATE CORALS. 



different parts of the world under the name of Montictilipora 

 petropolitaiia or Ckcstetes pctropoliiaims, 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 surface characters. 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 

 of the species have never been, so far as I am aware, either 

 described or figured. Even now it can hardly be said that the 

 position of our knowledge is absolutely satisfactory, seeing that 

 Pander's type-specimens have not been subjected to minute 

 examination. I am, however, indebted to the kindness of my 

 friend Dr Lindstrom for specimens from the Lower Silurian 

 of Sweden, the identity of which with the original Favosites 

 petropolitanus of Pander hardly admits of doubt, seeing that 

 they are derived from a corresponding geological horizon and 

 from a neighbouring geographical area. These specimens I 

 have subjected to a careful microscopic examination, and the 

 characters which they afford I regard as those specifically 

 diagnostic of M. peti^opolitana, Pand. Starting with this basis, 

 it is at once clear that the mere external form and surface- 

 characters of the corallum are of no use at all in the deter- 

 mination of this species. Thus all, or almost all, of the corals 

 which I have myself examined and described from the Lower 

 Silurian rocks of North America, and which I have identified 

 with M. petropolitana on account of their general form and 

 appearance, turn out to be widely different from the similar- 

 looking Russian species in their internal structure. I shall 

 also describe specimens from the Devonian which on merely 

 external examination would unhesitatingly be referred to M. 

 petropolitana, but which, tested by the facts of their minute 

 structure, are specifically or even sub - generically different. 

 Considering, therefore, that almost all the determinations of 



