BRITISH FOSSILS. 



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plates, or it may be regarded as composed of one plate broken up 

 into several elements. All the plates of the genital disk bear 

 scattered grannies. 



The primary spines are variable in size in the same and in 

 different specimens. They are shorter in young examples than in 

 old ones. In some before us, the largest spines are three times 

 as long as the diameter of the test. To the naked eye they seem 

 smooth, but under the lens are seen to be obsoletely and 

 finely striated longitudinally. They are more or less irregularly 

 angular ; the angles rounded ; in section they are irregularly 

 elliptical. They are broadest in their lower half, tapering upwards, 

 and terminating in a single, bifid, or trifid extremity. The ring 

 round tbeir bases is narrow, elevated, and strongly crenulated. The 

 neck is very sbort, and the margin of the articular socket is strongly 

 crenulated also. 



The secondary spines are very small, regular, round, and striated. 

 The dental-lantern is strongly developed, and often well pre- 

 served. It bears a close resemblance to that of Echinus. 



The following table shows the comparative dimensions of three 

 specimens selected on account of their differences : 





Elevated form. 



Depressed form. 



Ordinary 

 small example. 



Diameter of test 

 Height of test 

 Diameter of mouth 

 Diameter of apical disk 



1 inch. 



Of „ 

 0* „ 

 Of „ 



ly^Q- inch. 

 Oh „ 

 Oi „ 



Oy% inch. 



0 3 



The largest specimen in the Museum of Practical Geology 

 measures one inch and one-twelfth in diameter, and has spines 

 three inches and five-twelfths in length. The longest secondary 

 spine upon it measures one-fourth of an inch. 



Locality and Geological Position. The finest examples of this 

 beautiful fossil were found by William Buy, an acute collector, in 

 the^Forest Marble near Malmesbury and Chippenham. We have it 

 also^from the same rock at Hinton Abbey, where it was collected 

 by^Mr. Pratt. Mr. Bristow has found a tumid variety of it in the 

 Combrash near Wincanton. We have a group of specimens said 



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