150 



almost all those of the green sand, are siliceous, and are 

 rendered more interesting from the white quartz crystals 

 which fill their cavities, and from their excellent preser- 

 vation, by which the minutest markings on their surfaces 

 are shewn. 



If the fossil, which has been so frequently considered as 

 clipeus clunicularis, be not ranged under galeritesy Lam. 

 (conulus)^ it is in this formation that galerites first appears 

 in a very small species. 



Two of the genus cidarites are found in the neighbour- 

 hood of Heytesbury, which have been figured. Organic 

 Remains, Vol. iii. PL i. fig. 12 and 13, and are remarkable 

 for the appendages which are attached on the upper sur- 

 face, round the superior opening, and \vhich extend over 

 nearly a third of the surface. In one of these, fig. 12, this 

 appendage is formed of roundish plates, their margins having 

 obtained somewhat of a polygonal form, apparently from the 

 lateral pressure against each other from the increase of their 

 size : a foramen is generally discoverable in the centre of 

 each plate, and numerous short filaments connect them with 

 each other. Repeated examination of this structure, in 

 numerous specimens, has led to the suggestion, that this 

 appendage may have been formed by the young animals, 

 just escaped from their ovulae, and thus attached to the 

 surface of the parent. The fossil, represented fig. 13, is 

 furnished with appendages formed by raised lines, crossing 

 each other in an irregular trellised form. No explanation 

 has yet been offered which will account for the existence of 

 this structure. May it not have proceeded from corru- 

 gations of the cuticular surface occasioned by the attach- 

 ment of the young, as noticed in the preceding fossil? 



The chalk marl does not appear to contain any fossil 

 echinidae whatever. 



It is in the chalk, and chiefly, perhaps, in that with in- 

 terspersed flints, that these fossils are most abundant. 



