Ch. XXIV.] BRACKISH- WATER AND MARINE STRATA. 



493 



of France and Germany. ' It has been traced for thirty miles in a 

 straight line, and can be recognized at still more distant points. The 

 characteristic fossils are a small bivalve, having the form of a Cyclas or 

 Cymia, also a small entomostracan which may be a Oypris, or, if 

 marine, a Cythere (fig. 546), and the microscopic shell of an annelid 

 of an extinct genus called Microconchus (fig. 545) allied to Serpula or 

 Spirorbis. 



Fig. 545. 



Fig. 546. 



(^ a. Microconchus (Spiror- 

 bis) carbonarius. Nat. 

 size, and magnified. 

 b. Var. of same. 



Cy prist inflata (or Cy- 

 there t). Nat. size, and 

 magnified. Murchison* 



In the lower coal-measures of Coalbrook Dale, the strata, accord- 

 ing to Mr. Prestwich, often change completely within very short dis- 

 tances, beds of sandstone passing horizontally into clay, and clay into 

 sandstone. The coal-seams often wedge out or disappear ; and sec- 

 tions, at places nearly contiguous, present marked lithological dis- 

 tinctions. In this single field, in which the strata are from 700 to 

 800 feet thick, between forty and fifty species of terrestrial plants 

 have been discovered, besides several fishes of the genera Megalich- 

 tkys, Holoptychius, and others. Crustacea are also met with, of the 

 genus Limulus (see fig. 547), resembling in all essential characters 



Fig. 547. 



Fig. 543. 



Limulus rotundatus, Prestwich. 

 Coal, Coalbrook Dale. 



Glyphea ? dubia, Salter. 

 Syn. Apus dubius, Milne Edwards. 

 The oldest recorded decapod (or long-tailed) 

 crustacean. Coal-measures, Coalbrook Dale. 



the Limuli of the Oolitic period, and the king-crab of the modern 

 seas. They were smaller, however, than the living form, and had the 

 abdomen deeply grooved across, and serrated at its edges. In this 



* Silurian System, p. 84. 



