W. KEEPING ON THE DISCOVERY OF MELONITES IN BRITAIN. 395 



41. On the Discovert of Melonites in Britain. By Walter Keeping, 

 Esq., B.A., of the Woodwardian Museum, Scholar of Christ's 

 College, Cambridge. (Read March 22, 1876.) 



(Communicated by Professor Hughes.) 



In the appendix to a paper read before the Geological Society, June 9, 

 1875, I noticed the discovery of a large Urchin, nearly allied to 

 the American genus Melonites, from the Carboniferous Limestone of 

 Derbyshire, and preserved in the museum of the Geological Survey. 

 I have since been permitted to examine the specimen more carefully, 

 so as to determine it to be a new species of Melonites, a type of Echi- 

 noids characterized by its numerous ranges of ambulacral plates, which 

 until now was unknown beyond the boundaries of the New World. 



Another fragment, smaller, but well preserved and presenting all 

 the characteristics of the species, may be seen in the British Museum. 



Description of the specimens. — The larger specimen is a confused 

 mass of stout plates covering an area of 7| inches by 7 inches. 

 Among these may be seen some broad bands and broken masses of 

 another set of plates, which are each perforated by a pair of pores. 

 From the arrangement of this latter system of plates, converging, as 

 they do, roughly to the centre of the specimen, we see that this is 

 the remains of one large Echinoid broken up and much disarranged, 

 the larger scattered plates having formed its interambulacral areas, 

 the masses of smaller plates its ambulacral zones. 



The interambulacral plates (figs. 3, 4) are very numerous, and are all 

 about the same size, which shows that new series of plates were 

 intercalated to form the increasing circumference towards the equator 

 of the test, where there must have been as many as eight or nine * 

 ranges of plates. The plates themselves are very thick, and are 

 rather irregular in form, most of them being unequally six-sided ; 

 others are pentagonal, with one face rounded. These latter formed 

 the marginal ranges of plates, and they are marked with indenta- 

 tions for the ambulacral plates, with which they articulated. The 

 surfaces of all are ornamented with closely set minute tubercles. 



The plates were articulated with each other in a very irregular 

 manner, some of their edges being perpendicular to the surface, while 

 others are inclined as much as 30°, every intermediate stage being 

 seen. The marginal ranges share the irregularity so characteristic 

 of this Urchin ; for some of these plates are bevelled off from their 

 upper edges, others from the lower, so that in some parts of the 

 test the ambulacral areas overlapped the interambulacrals, in others 

 vice versa. 



The ambulacral areas are in better condition. When the test was 

 crushed their plates still cohered, and the areas were broken up into 



* Supposing, as the specimen seems to indicate, that the circumference of the 

 test was that of a circle with 3k inches radius, then, the ambulacral areas being 

 36 millims. broad (see infra), the interambulacral areas were 72 millims. broad ; 

 and allowing each interambulacral plate 8 millims., there were nine ranges of 

 such plates at the ambitus. 



