50 TABULATE CORALS. 



men of F. Gothlandica are wonderfully similar in size and 

 shape, being strikingly and often regularly pentagonal or hex- 

 agonal, and being further distinguished in thin sections by the 

 thinness of their walls (PI. I., figs, i and 2). In no case do the 

 larger calices assume the rounded character of those of F. For- 

 besi, nor is there the same conspicuous interpolation of small 

 tubes among the larger ones. All the forms which I have 

 here included under F. Gothlandica agree in the possession of 

 these regularly prismatic, thin-walled tubes ; and this being the 

 case, it remains to be considered if the size of the average 

 corallites can be properly considered as a specific character. 

 Most palaeontologists have answered this question in the affir- 

 mative, and have given names to species separated from the 

 typical F. Gothlandica principally or solely on account of the 

 average size of the corallites. Mr Billings, however, long ago 

 gave reasons for thinking that this character was one upon 

 which little stress could be laid (Canad. Journ., new sen, vol. 

 iv.), and my own researches have led me to entirely coincide in 

 this opinion. The examples which I have examined from the 

 Upper Silurian, and about the determination of which I enter- 

 tain no doubt, show conclusively the small weight that can be 

 attached to the mere size of the corallites. Those from the 

 Wenlock Limestone of Dudley, Longhope, and Benthall Edge, 

 have tubes of an average size of one line, some a little less, 

 some a little more (as stated by M'Coy, and also by Milne- 

 Edwards and Haime in their Monograph of the British Fossil 

 Corals). Those from the Wenlock Limestone of Gotland 

 (kindly sent to me and specifically determined by my friend 

 Dr Lindstrom) have usually a diameter of from one and a half 

 to two lines (agreeing, on the whole, with the measurements 

 given by Milne- Edwards and Haime in their work on the 

 Polypiers fossiles des terr. Pal.) ; but in one example the 

 tubes have a diameter of fully three lines (fig. 14, c). Lastly, 

 ■hose from the Niagara Limestone (Wenlock) of North Amer- 

 ica have mostly tubes of an average diameter of a line and a 

 half, this being sometimes reduced to a line or less, or increased 



