SILURIAN ROCKS. Sg 



The Eurypterida appear to have reached their 

 maximum development in the Upper Ludlow 

 period. There were probably more fishes living 

 then than arc yet known, since the Ludlow hone 

 bed, which is found all round the Woolhope area, 

 Mav Hill, and in many other localities, consists 



Of the remains of ti>he> matted togeth- 

 er, with fragments of the great Crustacea, some 

 its, and -'.me shells. Hie Downton sand- 



and Ledbury sha illy abound 



with the remains of Eurypterus and Pterygotus* 



With the Ludlow beds, trilobites become less 



important, and j which occur in the upper- 



St Ludlow bed- met with in the 



. onian r< - ks. rhe w ead crustacean 



type, Eurypterus sented in Scot- 



land. All the compound graptolites vanish, and 

 the group disappears with a few sin 



in the I I lldlow beds. Ludlow r<»ck> yield 



number -fishes, partly from 



from Ludlow. ( >n I 



is known from a multitude of S : and there 



ganisms known in the lower Pal&OZOic 

 tip ail fauna, with multi- 



tude . hydrozoa, 



crin trilobil 



phyllop irypterida, and group of 



mollusca, a- many fishes. It is in this 



od of time that tic 5," with flexible 



and elastic enveloping -hells make their first ap- 



rance in British rock-, in the genus Palachinus. 



v are of spheroidal form, and composed of 

 numerous plates in rows which overlap each other 

 obliquely at the edges. The group is always 

 scantily represented in the geological deposits, 



