DISTRIBUTION. 211 



in the lowest series, and the first two species have been found over a wide 

 horizontal area of distribution. 



Unfortunately, no species of Lamellibranchs seem to characterise any horizon 

 in the Carboniferous Limestone Series, including the Yoredale Limestones, but the 

 largest number of species occur in the upper beds of the division. 



In the Upper Carboniferous beds which succeed the Carboniferous Limestone 

 Series the Lamellibranchs afford the most definite and important zonal indices. 



The most important in this respect are Posidonomya Becheri, P. membranacea, 

 Posidoniella Isevis* P. minor, P. Kirhnani, P. variabilis, Pterinopecten papyraceus,* 

 Aviculopecten gentilis, A. Losseni, Pseudamusium fibrilloswm, Ghaenocardiola Footii. 

 Those species marked with * have a much wider vertical distribution than the 

 others. P. Becheri, it is true, does occur at a lower horizon, but its maximum is at 

 the lower part of the Pendleside Series. 



In the Coal Measures the distribution of several species of Anthracomya, 

 Garbonicola and Naiadites is so constant that I have been able to draw up a fossil 

 chart of the North Staffordshire coalfield for practical use by mining engineers in 

 ascertaining the horizon of beds in sinking and other mining operations. 



In the North Staffordshire coalfield several marine bands occur intercalated 

 amongst the freshwater beds containing Garbonicola, Anthracomya and Naiadites. 

 Two of the marine Lamellibranchs are of importance — Syncyclonema carboniferum,, 

 which occurs below the Grin Mine Coal ; Myalina compressa is an index of the 

 Marine bed 77 yards below the Moss coal. 



Of the species of Garbonicola and Anthracomya : 



Zone 1. A. calcifera is typical of the upper division of the Upper Coal Measures. 



Zone 2. A. Phillipsil with Garbonicola Vinti indicates the Black Band Series 

 immediately below Zone 1. 



Zone 3. The important shells in this zone are Garbonicola turgida, G. robusta, 

 G. subconstricta, Naiadites modiolaris and N. carinata, Anthracomya Wardi, 

 A. Adamsii, A. modiolaris, A. Williamsoni. 



The accompanying table of Life Zones in Carboniferous rocks (p. 212) has been 

 evolved from time to time. In it I have made use chiefly of Lamellibranchs and 

 Cephalopoda. It is found that certain species of the one group are accom- 

 panied by certain of the other. 



The most unsatisfactory part is that of the great Massif of Limestone, which is 

 characterised by Productus giganteus, probably about 3000 feet thick. I had 

 hoped by a careful examination of this formation, where it was split up into 

 separate beds, in the North of England and the South of Scotland, that the distri- 

 bution of fossils might have afforded some evidence that species characterised 

 certain definite horizons ; but this is not the case with any group of fossils which I 

 have collected and examined from those rocks. For the present, then, I cannot 



