ECHINITES. 461 



The Echinus is an animal of a roundish form, co- 

 vered with a kind of a shell or crust ; which in the 

 recent state is beset with rows of spines, and mark- 

 ed with rows of pores, disposed in avinuls. It 

 has a flat base, somewhat concave, in which the 

 mouth is always situated. The vent is variously 

 placed, and the whole family is divided into three 

 classes according to its situation : the anocystic, 

 which have the vent in the centre of the upper part. 

 The catocysti which have it in some part of the 

 base; and the pleurocysti, which have it in the 

 side or upper surface. Some species of all these 

 occur in this district. 



The cidaris papillata, the cidaris diadema, the 

 coranginum, and other's are found here and at Mai- 

 ton. 



SHELLS. 



Shells are divided into three classes, multivalves, 

 bivalves, and univalves ; the last class is subdivided 

 into unilocutor, having only one chamber or cavity 

 and multilocutor, having several chambers. The 

 multivalve class is scarce, containing only the lepas 

 or barnacle, and the chiton ; to which some add the 

 pholas, which besides two principal valves has small 

 accessary ones, upon the hinge and posterior slope. 



BIVALVES. 



These shells appear in immense numbers, many 

 of them corresponding with the recent marine pro- 

 duction. 



The petrified shells are generally much thicker 

 than the, recent ones, from which it appears that the 

 petrified ones have expanded in the process they 

 have undergone. 



All the shells we find in the limestone are not 

 petrified, but some of them are preserved. I had 

 an oyster shell from the rock on the top of Cock- 

 shot hill, near Kirkby-Moorside, retaining the ena- 

 mel on the inside of the valve, and appearing near- 

 ly as perfect as when it contained the fish in it. 



