﻿FROM THE LOWER CHALK. 



207 



one or two small primary tubercles ; at every third plate one of these is near the zonal 

 side (fig. 2 /), so that in each ambulacra there are two marginal rows of small perforated 

 and crenulated tubercles surrounded by several microscopic granules. 



The poriferous zones are very narrow, and have one pair of small round holes opposite 

 each ambulacra] plate, which are larger and more conspicuous on the dorsal than on 

 the ventral surface ; on the lateral and dorsal surfaces the pores are unigeminal, but at 

 the base, from the narrowness of the plates, they fall into double file (fig. 2 /), and near the 

 mouth-opening two rows go to each plate, the number of pairs of pores corresponding 

 with the number of the ambulacral plates ; taking the average as equal to seven ambulacral 

 plates for two inter-ambulacral an average-sized adult test would have seventy pairs of 

 pores in the lateral and dorsal portions of the zones ; the exact number at the base it would 

 be difficult to estimate, from the narrowness of the plates and the bigeminal arrangement 

 of the pores. 



The base is flat and the mouth-opening occupies the centre of the disc ; it is a small 

 obscurely decagonal opening (PI. XLVI, figs. 1 b, 2 b), equal in diameter to one fourth the 

 distance between it and the border ; the peristome is subcircular and divided into ten 

 equal lobes ; it is only in some rare specimens that this part of the anatomy of the test is 

 shown, as the oral opening is nearly always filled with closely adhering matrix. In 

 some fine specimens from the soft Grey Chalk I have been able to clear out the peristome 

 and demonstrate the oral lobes. 



The vent is proportionately very small, oblong, and acute at each extremity. It 

 occupies rather more than one fourth of the space between the mouth and border, 

 and is distant from the margin about its own long diameter (fig. \b). The plates of the 

 single inter-ambulacrum appear sharply incised by the vent, the margins of which are on 

 a level with them except at the inner extremity, where there is a bulging of the plates 

 extending to the peristome (PI. XLVII, fig. lb). 



The apical disc is well shown in my type-specimen ; and this structure is accurately 

 drawn in PI. XLVI, fig. 2 c. It is often prominent in consequence of the convexity of 

 its elements ; the five ovarial plates are of an irregular rhomboidal figure and closely united 

 together ; four of the five plates are perforated for the passage of the genital tubes, the 

 single plate is imperforate, and the microscopic madreporiform body occupies the entire 

 surface of the right antero-lateral plate. The five ocular plates, each having an orbit, 

 are small cordate bodies wedged into the angles of the ovarials, fig. 2 c shows this 

 structure magnified six diameters. Mr. Rones' very accurate drawing renders any 

 lengthened description of the discal elements unnecessary. 



In PI. XLVII, fig. 2 a, b, I have figured a very perfect mould of this Urchin to assist 

 the geologist to identify the species when all the test is absent. This mould is marked by 

 ten impressions made by the internal carinas of the test, which deeply groove the border 

 and base and extend from the lower part of the sides, pass along the floor of the test and 

 vanish at the peristome. In addition to these well-marked depressions Professor 



