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ECHINOCONUS 



times separated from each other by small accessory pieces ■ near the ambitus they 

 have a more regular shape, as seen in PI. L, fig. 1 d. All the plates are narrow, four or 

 five corresponding in depth to one inter-ambulacral plate. In the large specimen drawn 

 in PI. XLIX, fig. 3, there are fifteen plates in each dorsal inter-ambulacral column 

 between the angle of the disc, so that in this specimen, in the same space, there are 

 seventy-five plates in each half of an ambulacral area ; there are four rows of primary 

 tubercles in the widest part of the area, which have a zig-zag arrangement, the two 

 inner rows, which are the most irregular, disappear near to the middle of the base, and about 

 the upper fourth of the dorsal surface. The poriferous zones are very narrow, the pores uni- 

 geminal and oblique, six pairs being often opposite one large plate ; near the mouth they 

 form sets of triple oblique pairs, as in the example (fig. 5), magnified four times; from 

 several specimens before me in different stages of weathering, I have ascertained that a 

 small cuneiform plate is apportioned to each pair of holes ; this plate appears to have been 

 formed of two halves, so as to embrace the tubular feet ; the plates, therefore, of the 

 poriferous zones are not perforated for the soft parts, but have, in fact, grown around 

 them in the process of development. 



The inter-ambulacral areas are three times the width of the ambulacral at the 

 ambitus (PI. L, fig. 1 d) ; they are formed of large plates, each supporting from 10 to 14 

 primary tubercles, arranged in irregular horizontal rows, which sometimes assume an 

 hour-glass-shape-like disposition ; each of these tubercles is spiniferous (fig. 1 /), and 

 is encircled by a smooth depressed areola ; the boss is large, mammillated, and 

 crenulated at the summit, and the small round head is perforated ; those at the base are 

 larger, and form more regular horizontal rows ; when examined with an inch object-glass 

 they present the appearance shown in fig. 1 /. The inter-tubercular surface is covered 

 with rows of microscopic granules (fig. 1 d, c,f) ; the tubercles on the upper surface all 

 supported short spines, which are sometimes seen in situ, those at the base are large, 

 about one fifth of an inch in length ; they are slender, tapering, smooth, and covered with 

 fine longitudinal lines, which are delicately serrated near the summit, the stem is enlarged 

 at the base, where it is encircled by a crenulated collar (fig. 3). Dr. Bowerbank made a 

 microscopic examination of the spines of Echinocoui in his collection, and found that the 

 dorsal spines of E. conicus differed from those of E. subrotundits in being thickened at 

 the base, and set on tubercles as it were sessile, instead of being shortly pedunculated, as 

 they arc in the latter species. 



The minute moniliform spines of E. conicus, according to Dr. Bowerbank, are 

 Y^s of an inch in length, and of equal diameter (fig. 4). Three were accurately 

 measured, and all were as nearly as possible of the same size and proportions ; they were 

 all perfectly smooth. 



The mouth-opening is small and central (PI. L, fig. 1 b) ; the periostome is decagonal 

 (fig. 5), and armed with a dental lantern : this fact was first discovered by Mr. Charles 

 Stokes, F.G.S., and described by him in the 'Geological Transactions;' since that time 



