THE INTERAMBULACRUM. 67 



(1904, p. 3) criticizes this view and claims that there are more than two plates when tlie ventral 

 area is complete. I can only say that Miss Klem is mistaken. I have never seen an exception. 

 Two of the specimens here figured, showing this character of two plates ventrally, are from the 

 Hambach Collection which Miss Klem studied and herself figured, but it is fair to state that 

 they have been freed somewhat from matrix since she studied them. The specimens referred 

 to are Lovenechinus septies (Plate 45, fig. 1 = Miss Klem's Plate 2, figs. 5a-5d) and Melonechinus 

 muUiporus (Plate 57, fig. 1 = Miss Klem's Plate 3, figs. 6a-6d). In Palaeechinus quadriserialis 

 (Plate 30, fig. 3) there are two plates in the first row or zone, three in the second, four in the 

 third (excepting in one area, C), and above the introduction of the fourth column no more 

 columns are added, but all are continued to the apical disc. Loven (1874, p. 39) made the acute 

 observation that in Palaeozoic genera with multiple columns of plates, the adambulacral plates 

 alone are continued from the peristome to the apical disc. This observation holds true of 

 most of the family of the Palaeechinidae, as in Palaeechinus (Plate 30, fig. 3) and Lovenechinus 

 (Plate 41, fig. 1). The statement, however, needs modification. In those types where there 

 is a single plate at the ventral border, Perischodomus (Plate 64, fig. 2), obviously two columns 

 do not reach the peristome. Also in senescence (Melonechinus indianensis, Plate 53, fig. 1), 

 usually the two adambulacral columns drop out before reaching the apical disc; or in special- 

 ized types they may drop out very early, as in Hyattechinus beecheri (Plate 26) or H. pen- 

 iagonus (Plate 25, fig. 3). 



In the family of the Palaeechinidae the species have as a range from three to eleven columns 

 of interambulacral plates. In the several species the early development is as in Palaeechinus 

 quadriserialis, but in later growth additional differential characters are usually taken on. 

 In a young Lovenechinus missouriensis (Plate 40) a fifth column comes in soon after the fourth, 

 and in this specimen, which is very young and the only really young Palaeozoic echinoid yet 

 known, the fifth column originates near the mid-zone. In an older individual (Plate 41, fig. 1) 

 where many more rows of plates have been added dorsally, we find that the introduction of 

 the fifth column has been relatively shoved ventrally by the later added plates. In addition, 

 a sixth column is represented by one or two plates in three areas, E, G, I, but in two areas, 

 A, C, no sixth occurs. This occurrence of a column represented by only one or two plates is 

 very exceptional in Echini and suggests a disappearing part. In many specimens of the species 

 it is entirely absent. In Lovenechinus septies the method of introduction of columns is well 

 shown (Plate 45, fig. 1), as it is one of the best preserved specimens I have seen. In three 

 areas the two plates are present in the basicoronal row, restored in the other two; from this 

 point up or dorsally in the several areas the new columns come in quite regularly. In the 

 very choice specimen of Melonechinus muUiporus (Plate 57, fig. 1; text-figs. 245, 246) there 

 are two plates ventrally in areas G, I, one of the two in areas A and C, and they are restored 

 in areas where wanting, as indicated by dotted lines. Above this zone the introduction of 

 columns is shown without the absence of a single plate in all five interambulacral areas. 



