368 ROBERT TRACY JACKSON ON ECHINI. 



sion; width of the ambulacrum at the mid-zone, 24 mm.; width of the interambulacrum at 

 the same zone, about 26 mm. 



Ambulacra are wide, nearly equaling the width of the interambulacra, with eight columns 

 of plates at the mid-zone composed of a column of wide occluded, narrow demi-, and two columns 

 of somewhat irregular isolated plates in each half -area (Plate 52, fig. 7). Hambach says that 

 there are ten columns of ambulacral plates in this species. As I count the plates there are 

 eight columns, but in the irregular character of the ambulacra in Melonechinus, where there 

 are many plates, it is easy to count two more or two fewer columns in an area, according as one 

 includes or eliminates irregularities. The pore-pairs are in peripodia and lie on the outer por- 

 tion of each ambulacral plate. Small secondary tubercles and spines about 3 mm. long occur 

 on the ambulacral areas (Plate 52, fig. 10). The spines of these areas are perhaps a trifle longer 

 than spines on the interambulacral plates (Plate 52, fig. 11); but they certainly are not twice 

 as long as stated by Hambach and also by Miss Klem. In all species of this family the spines 

 of both areas are essentially alike, as far as known, and this species is probably no exception 

 to the rule. The slight differences observed may well be ascribed to conditions of preservation. 

 The ambulacrum ventrally has two columns of occluded and two of demi-plates (Plate 52, 

 fig. 9), but scattered isolated plates soon come in in the middle line of each half-area and repre- 

 sent the second stage in development. 



The interambulacra are a trifle wider than the ambulacra. At the mid-zone there are 

 six columns of plates in areas A, C, E, and G, but area I is imperfect, so that the number dorsally 

 could not be ascertained. Hambach says that there are five columns, but he apparently over- 

 looked an adambulacral column which in area C, one of the best areas, is sunken deeply in the 

 lateral furrow, and is in part wanting in this area. Interambulacral plates bear numerous 

 small secondary tubercles and spines, which, as preserved, are not quite so long as those in the 

 ambulacral areas (Plate 52, fig. 11). In area E the ventral portion is quite complete, but the 

 plates of the-basicoronal row are restored (Plate 52, fig. 8). There are three plates in the second 

 row, four in the third, and the fifth column comes in in the seventh row. In area C (Plate 52, 

 fig. 7) the fifth column originates in a pentagon, in the middle of the area, with a heptagon on 

 its left ventral border. Three rows higher up and well below the mid-zone the sixth column 

 originates with a pentagonal plate on the left of the center and with a heptagon on its right 

 ventral border. The specimen (Plate 51, fig. 6) is oriented by the introduction of columns and 

 ventral structure, and is reversed from the orientation given by Professor Hambach. Dorsally 

 the apical system is wanting. 



Lower St. Louis Limestone, Lower Carboniferous, St. Louis, Missouri. The holotype 

 and only known specimen is in Mr. Frank Springer's collection 8,116, and was from Professor 

 Hambach's collection. 



