PLATE 57. 

 Melonechinus multiporus (Norwood and Owen). Page 375. 



Figs. 1-3. Same specimen as photographs, Plate 55, figs. 1, 2. The most perfectly preserved specimen of a Palaeozoic 

 sea-urchin that I have seen, so that I am able to show it spread out by the Loven method with every plate in place 

 excepting a few which are restored in the basicoronal row of interambulacral areas A, C, and E, as indicated by 

 dotted lines (pp. 381-384). 



Fig. 1. Spread out to show five interambulacra and one ambulacrum complete from the basicoronal row to the apical disc. 

 Natural size. In ambulacrum J, also D, F, and H, there are four plates in a row ventrally. Passing dorsally in J 

 are found in addition dissociated isolated ambulacra! plates in the middle of each half-area, then isolated plates 

 become more frequent and are continuous, thus making three columns of plates in a half-area; next more isolated 

 plates appear, making four columns of plates in a half-area; then still more series of isolated plates originate, 

 making five columns in a half-area at the mid-zone, which is the specific character. Above the mid-zone we soon 

 pass dorsally into an area with four plates in a row to a half-area, higher up only three plates in a row, and near 

 the ocular plate only two plates in a row in each half-area, and in actual contact with the ocular there is only a 

 single primary plate in each half-area, a simpler condition than that seen in the youthful plates ventrally (compare 

 text-fig. 237, p. 231). In ambulacrum B there are only two plates ventrally, enlarged in fig. 3; this character of 

 primary plates occurring in the ventral zone is a unique regressive variation for the genus as far as known. The 

 ambulaeral areas as in this figure are shown more enlarged in text-fig. 245, p. 382. 



In interambulacra I and G there are two plates in the basicoronal row, but the plates of this lowest row are partly 

 or wholly restored in the other three interambulacral areas, as indicated by dotted lines (compare text-fig. 245, p. 382). 

 In all interambulacra the initial plate of column 3 is a hexagonal plate in the second row. In interambulacra A, 

 E, and I the initial plate of column 4 is a pentagon in the third row on the right of the center, but in areas G and 

 C this plate of the column 4 is on the left of the center. In interambulaerum I the initial plate of column 5 is a 

 pentagon in the center in the seventh row, with a heptagonal plate on its right ventral border. In area G, penta- 

 gon 5 is in the sixth row with the heptagon on its left ventral border. In area E, pentagon 5 is in the seventh row 

 with the heptagon on its right ventral border. In area C, pentagon 5 is on the left of the center in the sixth row 

 with the heptagon on its right ventral border, and in area A the pentagon 5 is in the center in the sixth row with the 

 heptagon on its right ventral border. In interambulaerum I the initial plate of column 6 is a pentagon on the right 

 of the center in the tenth row with a heptagon on its left ventral border. In area G, pentagon 6 is also on the 

 right of the center in the tenth row, but the heptagon is on its right ventral border. In areas E and A pentagon 

 6 is on the left of the center in the tenth row with the heptagon on the left ventral border. In area C, pentagon 

 6 is also on the left of the center, but in the ninth row with the heptagon on its right ventral border. In inter- 

 ambulaerum I the initial plate of column 7 is a pentagon in the center with a heptagon on its right ventral 

 border. The initial pentagon of column 7 is in the center in all other areas with the heptagon on its right 

 ventral border in area E, but on its left ventral border in areas G, C, and A. The seventh column originates 

 in the fourteenth row in areas I and G, in the fifteenth row in area K, and in the thirteenth row in areas C and A. 

 In interambulaerum I, the initial plate of column 8 is a pentagon on the right of the center, with a heptagon 

 on its left ventral border. The initial pentagon of column 8 is on the left of the center in areas G, E, and 

 A, but by rather rare exception in area C the initial plate of column 8 is a tetragonal plate, on the left of the 

 center with two adjacent heptagons to compensate for its two sides wanting. The heptagon next to initial 

 pentagon 8 is on its left ventral border in areas I, E, and G, but on its right ventral border in area A. The eighth 

 column originates in the 18th row in areas I and C, in the 19th row in area G, in the 21st row in area E, and in the 

 17th row in area A. (Compare text-tigs. 25, p. 70; 245 and 246, p. 382.) Dorsally in each area all the columns of 

 interambulacral plates continue to the' apical disc, but are strung out dorso- vent rally as is usual in that area (pp. 

 378, 381-384). 



Fig. 2. Dorsal area. X 4. Shows ambulacrum J, with two plates next to the ocular and the number of plates in a row 

 increases passing ventrally. The young interambulacral plates are in contact with the lateral borders of ocular J 

 on either side. Oculars are all imperforate and reach the periproct. Genital G has four pores; all other genitals 

 have three pores each. 



Fig. 3. Ventral part of ambulacrum B. X 4. As a radial regressive variation, this area has a few primary plates extending 

 across the half-area. Above these primary plates are demi- and occluded plates, which is the usual character of the 

 ventral ambulaeral plates in the genus. Primary plates are characteristic of Palaeechinus at the mid-zone, and also 

 are characteristic as a ventral developing stage in Lovenechinus (Plate 42, fig. 1) and Oligoporus. For such primary 

 plates to occur ventrally in Melonechinus is a case of extreme radial regressive variation and is a unique variant 

 for the genus (pp. 19, 228, 360, 381). 



