ECHINITES. 205 



the pores are concealed : but in general we may see the pores on the 

 surface, at least Avith the help of a glass. Where the surface, or 

 rather any thick edge, has been long exposed to the weather, it dis- 

 plays a singularly cancellated, or reticulated appearance ; a multitude 

 of cross markings, which are but faintly seen in the recent fracture, 

 being then conspicuous. This is the case in part of the specimen 

 given in Plate VI, Fig. 12, which is from Silphoue. This fossil is 

 also remarkable, in having been penetrated by the pholas and the 

 teredo. If we consider this substance as a zoophyte, its innumerable 

 pores might well entitle it to be called a millepore. The pores, how- 

 ever, are not of a determinate shape, and the hair-like fibres, which 

 may often be detached singly from the mass, vary in their form as 

 well as their size. The substance is calcareous, effervescing strongly 

 with acids. It bears a resemblance to petrified wood ; and might have 

 been considered as such, had the fibres been differently disposed.* 



The Echinus, sea-urchin, or sea-egg, is so closely allied to the 

 zoophyte family, that it seems proper to introduce here the petrified 

 remains of animals of this genus, known by the name of echinites. 



The echinus is an animal of a roundish form, covered with a 

 kind of shell or crust, generally thin, which in the recent state is 

 beset with rows of spines, and marked with rows of pores, disposed 

 in avenues. It has a flat base, often 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 anocysti, which have the vent in the vertex, or centre of the 

 upper part; the catocysti, which have it in some part of the base; and 



* The screw'Stone, which occurs in the oolite, might have been noticed among the 

 zoophytes. It is evidently the cast of the encrinite column, the body of the column having' 

 been dissolved. The entrochite limestone, which is found in blocks in the alluvium, owes its 

 peculiar feature to fragments of encrinite columns. 



3 E 



