340 ME. HERBERT BOLTON ON [Aug. I9II, 



more than the forms now recorded from the Bristol Coalfield, that 

 marine phases are not restricted to the Lower Series, but pass 

 upwards to near the top of the Middle Coal- Measures in Yorkshire 

 and into the Farrington Series of the Upper Coal-Measures in the 

 Bristol and Somerset area. 



Further, it is evident that the marine fauna undergoes no marked 

 change throughout — forms common in the Lower Coal-Measures 

 ranging upwards, and not being replaced by new species. This lack 

 of mutability will probably prevent the use of marine bands for 

 zonal purposes. The continuity of life-forms in the Middle Coal- 

 Measures of Yorkshire is strikingly complete, less so in the Bristol 

 Measures ; but even here the long interval represented by tho 

 deposition of over 1.000 feet of Pennant Grit has not sufficed to 

 prevent their recurrence at higher levels. 



The presence of fragments of insect-wings in the Lower Coal- 

 Measures at South Liberty Colliery, Bristol, is of interest and 

 suggestive, since- insect-wings are also known in the South Wales 

 Coalfield, 1 which must originally have been joined up to the Bristol 

 Coalfield. 



It may not be inopportune to put on record my conviction that 

 the three Coal-Measure genera, Carbonicola, Naiadiies, and Antlira- 

 comya, will yet prove to be characterized by only a few definitely 

 marked forms worthy of specific rank, and that around each of such 

 species it will be possible to group a number of variants, some of 

 which are now regarded as valid species. It is quite possible to 

 select two or more types, and between such selected forms to place 

 a graduated series which effectually bridges the interval, so that 

 one selected species seems to pass by a series of stages into another. 

 This is easily done with several of the present species of Carbonicola, 

 which can be thus joined up with C. aquilina and C. acuta. 



The power of variation or mutability (in De Vries's sense) 

 possessed by these genera would seem to have been very great, or 

 their response to the conditions of environment very ready. I am 

 also inclined to the belief that dissimilar and relatively fixed species 

 occurring at widely separated horizons have produced under the 

 stimulus of environment, or of some other cause, a series of variants 

 or mutations precisely similar in outward form and character. 



EXPLANATION OF PLATE XXVII. 



Fig. 1. Estheria tenella Jordan. Magnified 3 diameters. Koof-shales over 

 the High Vein, Coalpit-Heath Colliery. (See p. 324.) 



2. Estheria tenella Jordan. Portion of a clustered mass showing the 



general mode of occurrence. Natural size. Same horizon and 

 locality as fig. 1. 



3. Estheria cf. tenella Jordan. In shale between the Ashton and 



Bedminster Great Veins, South Liberty Colliery, Bristol. A smaller 

 and more oral form than the Coalpit - Heath type, magnified 

 3 diameters. (See p. 325.) 



1 Q. J. G. S. vol. lxvii (1911) pp. 149-74 & pis. vii-x. 



