250 A HISTORY OF LONGEIDGE. [Chap. 12. 



This accumulation took place during a period of gradual depression, 

 due to secxilar cooling and consequent contraction of the earth's crust, 

 ■which continued, probably, but with interruptions, during the subse- 

 quent deposition of the Yoredale Eocks and the Millstone Grit. 



The inhabitants of this vast Carboniferous Sea were numerous and 

 prolific. More than two thousand have been named. ^ Of crinoids 

 alone more than one hundred species have been found. In the quarry 

 at ThorTiley the stems and plates of these curious marine organisms 

 may be found in abundance. The so-called heads or bodies f calyces J 

 also occur, but are far more rare than the stems. The presence of a 

 larger proportion of animal matter may have led to a more rapid dis- 

 integration or disarticulation of tliis part of the animal. (See plate II. 

 fig- 4.) 



The crinoids were animals of a low organisation, and were fixed to 

 the bottom of the sea, during a greater part of their existence, by a 

 flexible stem, composed of centrally perforated calcareous discs placed 

 one above another. The so-called head consisted of a globular or 

 pear-shaped arrangement of accurately-fitting hexagonal plates, and 

 was furnished with five long, branching, feathery arms. The upper 

 surface of these arms, and their branches, carried grooves covered with 

 cilia, the continual vibration of which produced rapid currents of water, 

 bearing oxygen and small living organisms, in the direction of the 

 centrally-situated mouth. 



The Crinoidea are now represented by only a few small species. 

 These are found only in tropical seas, and are gradually becoming 

 extinct. Their fanciful resemblance to plants, their stony structure, and 

 their fixed habit, has earned them the name of " Stone Lilies." 



Various species of coral may also be found in the Thornley lime- 

 stone. The most frequent are Zaphrentis, ClisiophyUum, and 

 Fenestella ; the latter appearing like very delicate lace or network. 



The presence of these organisms indicates a far higher temperature 

 of sea-water than now obtains in the British area, and this high 

 temperature was probably a principal agent in the wonderful develop- 

 ment of plant-life that followed, so soon as the land emerged from 

 the waters. 



'Prestwich. 



