PROM THE UPPER GREENSAND. 243 



Greensand fossils, has satisfied me, after a comparison of specimens in my collection from 

 that locality with a series of type tests from the Upper Greensand of Wilts and Dorset, that 

 the two Urchins appertain to the same species, and that Lamarck's name ought to be 

 retained. Catojiygus columbarius, it is true, assumes a considerable variation of form as 

 regards the elongation, shortening, height, breadth, and inflation of the test, so that there 

 is field enough for species-makers, who attach undue importance to these characters, to make 

 several varieties out of a handful of specimens. These phases of form appear to me to 

 have depended on the physical conditions which surrounded the life of the Urchin, and 

 have nothing whatever to do with the specific characters I have pointed out in my 

 diagnosis of the species. 



The test is ovate or subrotund and always wider behind than before; the dorsal 

 surface is tumid, varying in the degree of its elevation ; in some specimens it is sub- 

 depressed and declines anteriorly, in others it is subconic and much elevated in the centre, 

 the true apex being almost the apical disc, whereas, in general, that body is excentral and 

 situated before the vertex, which is formed by the ridge of the single inter-ambulacrum. 

 The sides are rounded and more or less inflated, and the posterior extremity is truncated 

 more or less abruptly. A more or less developed obtuse central elevation extends along 

 the ridge of the single inter-ambulacrum to the upper border of the vent, where it forms 

 in many examples a prominent apicnlated arch over the periprocte, PI. LV, fig. 2 d. In 

 all the specimens I have examined this prominence exists, but its degree of development 

 varies much. The ambulacral areas are narrowly lanceolate, limited to the dorsal surface* 

 subpetaloid, and very uniform in their proportions in all the varieties. The single area 

 and anterior pair are nearly equidistant from each other, but the posterior pair are more 

 distant from the anterior pair, and are placed much closer together than the others and extend 

 backwards. The number of pairs of pores in each zone is nearly equal, varying from 

 twenty-eight to thirty in well-grown adult shells ; the pores in the outer row are elongated 

 and oblique, and in the inner pores are round and appear to be conjugated by fine 

 olilique sutures. At the lower part of the petals the pores become smaller, and are set 

 much wider apart as they pass round the border of the test and extend to the peristome. 

 The ambulacral plates are narrow in the petaloid portion of the zones, and become 

 much larger and broader beyond the petals ; each plate has its pair of holes which 

 can be distinctly traced in good specimens, and they form the true poriferous zones on 

 the sides and base of the test. PI. LV, fig. 2 h, c, d, <?, shows these poriferous zones. 

 Around the mouth the pairs of pores again form petals as on the dorsal surface, and they 

 are here so arranged that they form ten short petaloidal ambulacra, forming rosettes 

 around the mouth, and constructed like those on the dorsal surface. Fig. 2 h shows this 

 structure extremely well in a drawing magnified six diameters. 



The wide inter-ambulacral areas arc formed of large oblong plates, the surface of 

 which, as well as those of the ambulacra, are covered with minute moniliform tubercles, 

 interspersed with microscopic granules. Pig. 2 g shows the arrangement of the tubercles 



