Vol. 68. J LOWER CARBOXIFEROtrS OF NOETH- WESTERN ENGLAND. 453 



Although the general sequence tabulated in fig. 2 (p. 452) is appli- 

 cable throughout the whole area under description, the detailed 

 characters of certain portions of the succession vary in different 

 parts of the area, so that parallel sections of districts situated far 

 apart present at first sight considerable differences. When, how- 

 ever, the same levels are traced through the intermediate districts, 

 the continuity of the faunal horizons can, as a rule, be readily 

 established. For this reason two species have occasionally been 

 cited as the indices for a particular band, since, although the two 

 may occur together over a portion of the area, sometimes one and 

 sometimes the other species may characterize the deposit when it 

 is traced laterally into outlying districts. 



For example, the band which lies near the summit of the Lower 

 Dihunojjhyllum Sub-zone is characterized in the Grange, Arnside, 

 and Purness districts by an abundance of Cyrtina septosa^ together 

 with somewhat rarer examples of CJwnetes aff. comoides ; while in the 

 Shap District, at the same horizon, Ohonetes aff. comoides occurs 

 abundantly, but Cijrtina septosa is locally rare. 



Any change in the fauna of any particular horizon when traced 

 laterally is found to be accompanied, as a rule, by a corresponding 

 lithological change; indeed, the fauna appears often to depend directly 

 on the lithological character of the deposit, and one or more species 

 may persist in a particular district into a higher horizon than is 

 usual elsewhere, if the lithological character of the deposit remains 

 unchanged. Thus Cyrtina carhonaina, which in the Kendal 

 District is practically limited to its own sub-zone, is found in the 

 Ravenstonedale District to persist into the base of the Lower 

 Dihunopliyllam Sub-zone. This local survival of C. carbonaria 

 appears to be t^lie direct result of the shallow-water phase, 

 which in the Eavenstonedale District continued into the Low^er 

 Bihunopliyllum Sub-zone, while elsewhere this sub-zone is co- 

 incident with a period of increased submergence. A good example 

 of prolonged existence may also be cited from the same district, 

 where the entrance of the typical fauna of the Lower 

 DibunopJiyllum Sub-zone is delayed, owing to the abnormal 

 j)ersistence of shallow-water conditions. 



Another instance, of the influence exerted by the depth and 

 character of the bottom on the distribution of a species, is found 

 in the case of Zaplirentis ennishilleni. This species, which appears 

 to have flourished only in shallow water, and preferably on a bottom 

 composed of argillaceous material, enters at different horizons in 

 the different districts. Thus, in the Arnside and Kendal Districts, 

 it enters near the summit of the Miclulinia Zone; in the Shap 

 District it is found at the base of the Lower Productus corrugato- 

 hemisphericus Zone, after the entrance of Lithostrotion ; while, in the 

 Eavenstonedale District, it occurs with several varieties near the 

 summit of the Ashfell-Sandstone Series, immediately below the base 

 of the Cyrtina-carhonaria Sub-zone. 



"A good instance of the association of a particular species with a 

 definite type of deposit is afforded by Syriwjotliyris cuspidata. 



