BRITISH FOSSILS. 



3 



genital plates, of which that combined with the rather conspicuous 

 madreporiform body is largest, and of five perforated oculars, of 

 which the two antero-laterals are much larger than the others ; it 

 is the size and contiguity of these plates that chiefly cause the 

 elongation of the disk. The fifth or posterior genital plate is 

 minute and imperforate. On the cast there are traces of the pre- 

 sence of an internal sand-canal. The test is remarkably thick in 

 substance, especially beneath. 



The following table of the comparative dimensions of six spe- 

 cimens selected on account of their difierences will serve to illustrate 

 the variation of proportions in this species. 



No. 



Locality. 



Stratum. 



Length. 



Breadth. 



Height. 



1 



2 

 3 

 4 

 5 

 6 



Dover 



Chaldon 

 j> " " 

 >i " ~ 

 j> " " 



Chalk Marl 

 Lower Chalk 

 Chloritic Marl - 



» j> 

 >? J? 



2 



m 



2 



1 1 1 

 2 



110 

 1 3 

 1 



lA 

 lA 

 lA 



Varieties. There is a curious variety of this species, apparently 

 an abnormality, in which the anterior sulcus is impressed into a 

 deep and almost acute furrow. I have found it at the junction of 

 the Chalk Marl and Upper Greensand near Abinger in Surrey, 

 and there is a very fine and large specimen of it in the British 

 Museum. 



Occasionally casts in Greensand are found extremely compressed. 

 One of these appears to be the Spatangus Murchisoni of Konig, 

 Icones Sectiles, £ 169. The figure of Spatangus hemisphericus 

 of Phillips' Geology of Yorkshire, pt. i, pi. 1, fig. 15, bears a close 

 resemblance to this species also, partially compressed. The " Spa- 

 tangus argillaceus " from the Speeton Clay, of the same work, is 

 evidently a Holaster, and possibly this species. In cabinets 

 specimens of a Hemiaster from the Gault are often so labelled by 

 mistake. 



Locality and Geological Position. This urchin abounds in the 

 bed of Chalk with green grains intervening between the Upper 

 Greensand and Chalk Marl. It is found also plentifully in the 



