138 GUIDE TO THE FOSSIL INVEETEBEATE ANIMALS. 



Gallery beds of Beer, in Devonshire, and Westbury, near Bristol. 

 VIII. AviciUa coiitorta is the best known, as giving a name to a 

 widely distributed horizon of Ehaetic Age. Like the Khaetic 

 shells generally, it is relatively small, perhaps in conse- 

 quence of brackish water. Monotis dexussata and Chlamys 

 valonicnsis are also important. There is a small but inter- 

 esting series from the Keuper Marls of Warwickshire. The 

 Conchylian Age has no shell-bearing rock in this country. 

 Table-case Permian. This Epoch is represented by marine shells 

 from the Magnesian Limestone of Durham and the red marls 

 near Manchester. Note Monotis spelnncaria and Byssoarca 

 striata from the former, and the tiny Rissoa and Turho from 

 the latter. Bakevjellia antiqiia comes from both localities, 

 and from Tyrone as well. 



h 



Fig. 73. — Recent and fossil shells of Pleiirotomm-ia. a, P. Quoyana, now 

 living in the West Indies ; b, P. platyspira from the Middle Lias of 

 France. The slit s receives the projecting anus, and, as the shell 

 grows forward, is filled up by shell-substance. Both figures are less 

 than natural size. 



Table-ease Carboniferous. The shells of this Epoch come mainly 

 from the Coal Measures and the Mountain Limestone, the 

 rocks of Middle Carboniferous or Moscovian age having 

 yielded few mollusca in this country. The Coal Measures, 

 though largely of fresh or brackish water origin, contain 

 many marine bands ; the Lower Carboniferous rocks are all 

 marine. The fossils have not here been separated according 

 to age or rock or habitat. It will, however, be noticed that 

 the Coal Measure fauna, and particularly the freshwater 

 elements in it, occur among the lamellibranchs, whereas the 

 gastropods are almost all from the Mountain Limestone. 

 Among the Coal Measure fossils are many described by 

 Sowerby in Prestwich's classical memoir on Coalbrookdale, 

 and many described by Dr. Wheelton Hind in the Mono- 

 graphs of the Palaeontographical Society. The fresh-water 

 forms include Anthracomya and Carhonicola \_Anthracosia^ 



