FROM THE CHLORITIC MARL. 115 



expanding, however, near the mouth, where an increased number of holes seem ahnost 

 completely to encircle the peristome (fig. 1 h) ; throughout the zones the pores are very 

 regularly bigeminal, and are grouped into distinct ranges, the small plates forming these 

 zones being beautifully dovetailed into each other (figs. 1 d, g, h) ; by this arrangement 

 there are from six to seven pairs of holes opposite each ambulacral plate, which gives 

 3G X 6 = 216 pairs of holes in each zone. 



The inter-ambulacral areas are rather more than double the width of the ambulacral ; 

 the individual plates are long and narrow, being only a little deeper than those of the 

 latter ; two rows of tubercles occupy the centre of the plates, and extend very regularly in 

 size and disposition from the peristome to the disc ; on the zonal side of this central row 

 a second row extends from the peristome over three fourths of the area, and between the 

 central row and the median suture there is another row of the same length ; in addition 

 to these six rows a few additional tubercles are introduced at the zonal and median sides 

 of the widest ambital region of the area. I have shown this chaiacter in figs. 1 (/ and h, 

 and the gradual disappearance of the tubercles in the upper part of the area is seen in figs. 

 1 a and c, and in its lower part in figs. 1 b and h ; between the tubercles a number of 

 small granules are sparsely distributed over the surface of the plates. 



The tubercles in this genus are very uniform in size and structure throughout both 

 areas ; the boss rises suddenly from the surface of the plate, without any areolar depres- 

 sion ; its summit is very finely crenulated only where it closely embraces the mammillon, 

 which is small, prominent, and perforated at the summit ; in the widest part of the 

 areas the granules form imperfect circlets around the tubercles, and an increased orna- 

 mentation at the ambitus and base ; at the iipper surface they become more sparse in the 

 ambulacra (fig. 1 d), and are almost entirely absent in the inter-ambulacra (figs. 1 a, c). 



The mouth-opening is nearly circular, about one third the diameter of the test (fig. b) ; 

 the peristome is slightly incised and vmequally divided, the ambulacral being much longer 

 than the inter-ambulacral lobes, allo\Aing a wider space for the development of pedal pores 

 around the peristome. 



The apical disc is absent, and the upper part of the test is unfortunately broken off, 

 so that no indication of the size or form of the disc remains imprinted on the mould. 



Affinitws and Differences. — The only two species of this genus at present known 

 are Pedinopsis Meridanensis, Cotteau, from the Neocomian of Aude ; and P. Wiesti, 

 Wright, from the Chloritic Marl near Chardstock. These Urchins resemble each other 

 very much ; P. Meridanensis (PI. XXIX a, fig. 2^) has a greater number of tubercles ; 

 and they are likewise larger and more regulai'ly disposed on the plates ; the poriferous 

 zones are wider, and the holes larger than in P. Wiesti. The bigeminal character of the 

 zones, the smallness of the tubercles, and the thinness of the shell, are special characters 

 by which this Urchin can be readily distinguished from all other congeners. 



1 Copied from M. Cottcau's ' PaMontologie Franfaise,' pi. 112.j, for comparison witli tlie English 

 species. 



