194 R. T. JACKSON — STUDIES OF PAL^ECHINOIDEA. 



lacra on either side. The adambulacral plates are inclined under the 

 adjacent ambulacrals, as in Melonites muUiporus (plate 2, figure 5) ; other- 

 wise there is no imbrication, but interambulacral plates are inclined 

 slightly upward and outward on all sides, as usual in hexagonal plates 

 of this group. 



At the ventral end of this specimen, as far as preserved (plate 6, figure 

 31), there are 6 columns of plates, the left adambulacral being produced 

 mentally. The adambulacral columns are composed of pentagonal, the 

 median of hexagonal plates, as in Melonites. In the second row the 

 seventh column is introduced by pentagon 7. It has an equal number 

 of columns on either side, and has a heptagonal plate on its left ventral 

 border. This heptagon is on the wrong side, as by the law of symmet- 

 rical development it should appear on the right border of pentagon 7 ; 

 but such variations are quite frequent, as described under Melonites. In 

 the fifth row above initial plate 7 the eighth column is introduced by 

 pentagon 8, with a heptagon on its right. Column 8 has 3 columns on 

 the left and 4 on the right, so that it is a left-handed series of plates, and 

 also the heptagon is on the wrong side. In the sixth row above penta- 

 gon 8 the ninth and last column added is introduced by pentagon 9, with 

 a heptagonal i)late on its left. This plate by rule should be on the right 

 of pentagon 9. This ninth column at its point of origin has 4 columns 

 on the left and 4 on the right, its correct theoretical position. 



At this horizontal plane and just above it there is one of the most 

 irregular and unusual arrangements of plates seen in any Palseechinoid. 

 Next to pentagon 9 on the left there is an accessory pentagon, P, which 

 is a member of column 8, and to compensate for its missing side, as it 

 should be a hexagon, tliere is a seventh side added to a plate, H\ next 

 it on the left, which he])tagonal plate is a member of column 5. The 

 second plate, 9', in the ninth column, is separated from the initial plate 9 

 by the interposition of 4 plates, 7', 7' and 8', 8', belonging to the two rows 

 succeeding pentagon 9, and tliese 4 plates are respectively members of 

 columns 7 and 8, as indicated by tlieir numbers. One other case has 

 been observed in wliich the initial plate of a column was separated from 

 the second plate of its series, namely, that shown in the ninth column 

 of Melonites gignntevs (plate 5, figure 21), but in this case the succeed- 

 ing plates of column 9 are only separated by the intercalation of one 

 pair of plates, and the initial plate is tetragonal instead of pentagonal.* 

 These are the only cases observed in the numerous specimens of Pala?- 

 echini studied in which the successive plates of columns were not con- 



* Still a third case of the separation of successive plates in a column by the intercalation of the 

 lateral plates of adjacent columns is shown in the specimen of Oligoporus missouriensis (plate 9, 

 figure 50), and a fourth case is seen in Rhoechinus gracilis, in the seventh column of area /(plate 7, 

 figure 36). 



