228 ME. L. D. STAMP ON THE HIGHEST [vol. lxxiv, 



It thus appears that the Aymestry Limestone does not thin and 

 die out to the west of Mocktree, but that there is a true lateral 

 transition into calcareous shales. The characteristic fauna of 

 brachiopods and corals certainly does die out somewhat abruptly 

 towards the Avest, and it was most probably of the nature of a 

 littoral fauna thriving round the island of Central England in 

 Silurian times. 



(3) The Upper Ludlow Group. 



The Upper Ludlow Group in this district admits of the same 

 broad divisions as in the Ludlow district : — 



D. Chonetes Beds — zone of Chonetes striatella. 



C. Rhynchonella Flags — zone of Rhynchonella nucula. 



C. The Rhynchonella Flags. — The series has an ill-defined 

 lower limit, since the striped Dayia Shales pass up gradually into less 

 conspicuously-laminated shales containing Orthoceras striatum, 

 O. ibex, and numerous small lamellibranchs. These are in turn 

 succeeded by hard, massive, calcareous beds containing few fossils, 

 followed by calcareous flagstones, 150 to 200 feet thick, which 

 become more fossiliferous in the upper part. The flagstones 

 break up in rough slabs, fairly distinct from the more nodular and 

 irregular weathered masses of the Chonetes Beds above. Certain 

 bands in the flagstones, generally of a blue colour, are harder and 

 more massive than the normal, and are much used in local 

 buildings. 



Fossils are not very abundant in the Rhynchonella Beds. The 

 commonest are casts of a few chambers of an Orthoceras. These 

 examples are generally about an inch across, and might easily be 

 mistaken for inorganic concretions, except for the presence in most 

 of an indication of the siphuncle almost in the centre of the 

 chamber. Several specimens of these in the Geological Depart- 

 ment, British Museum (Natural History), are labelled ' cast of 

 septal surface of Orthoceras marloense't J. Phillips.' Though 

 occurring also in the Chonetes Beds, these casts are very typical of 

 the Rhynchonella Beds, especially when associated with Serpulites 

 lonyissinms, Orthonota amygdalina, and with large specimens of 

 Rhynchonella nucula. 



The upper limit of the Rhynchonella Beds is generally marked 

 b}'' several harder blue bands, and by the appearance of Chonetes. 



D. The Chonetes Beds. — The Chonetes Beds fall naturally 

 into an upper and a lower division. 



(a) The Lower Chonetes Beds are irregularly-bedded calcareous 

 flagstones of blue-grey colour. They weather into thick slabs of a 

 nodular character, due to the somewhat concretionary nature of 

 the beds, and thus differ from the Rhynchonella Flags. 



