240 R. T. JACKSON — STUDIES OF PALvEECHINOIDEA. 



b}^ a process of drawing out (page 189). The simplest species has 4 col- 

 umns of interambulacral plates and the highest species has 9 columns^ 

 with a large number of species which are progressively intermediate in 

 structure. 



Melonites (see figure 1, page 164; plate 2, plate o, figures 10-17, plates 

 4 and 5) is the best known of all Paleozoic genera, one species, M. multi- 

 porus, Norw. and Owen, being abundantly represented. It is the study 

 of this genus that gave the key to the ])resent classification. This genus 

 is the most highly differentiated in the direct line of progressive develop- 

 ment of both ambulacra and interambulacra of any of the Paleozoic 

 Echini.* The ambulacra have ventrally 4 columns of plates (see page 

 140), which are comparable anatomically and morphologically to the 4 

 columns of ndultOllf/oporus (figure 1, page 191). During development new 

 columns of plates are introduced between the lateral and median columns 

 of each half ambulacrum (plate 2, figure 4), until the full complement 

 of the species is attained. The species M. dispar, (Fischer), has 6 col umns 

 of ambulacral plates, and on this basis, as well as having the least num- 

 ber, 4, of interambulacral plates, is the simplest species of the genus. In 

 its adult it rei)resents a very earl)^ stage in the development of higher 

 species (plate 2, figure 2, and plate 5, figure 21). The two species, M, 

 indianensis, Miller and Gurley, and M. septenarius, Whitfield, have each 8 

 columns of ambulacral plates in the adult. The latter species has 7 col- 

 umns of interambulacral plates (plate 9, figure 49), and is therefore con- 

 sidered the more specialized of the two. The next species, M. crntisus^ 

 Hambach, has 10 columns of ambulacral and 5 of interambulacral plates. 

 As the ambulacra are considered the more important character, this species 

 is placed above M. septenarlit^, although that species has 2 more columns 

 of interambulacral plates. .1/. Irreguhirk^ Hambach, has from 6 to 10 col- 

 umus of ambulacral and from 5 to 7 of interambulacral plates. f The- 

 next species, M. iiudtiporas, Norw. and Owen, which is the most abundant 

 of all Paleozoic echinoids, has 10 columns of ambulacral plates and 7, 8- 

 or 9 columns of interambulacral plates. This variation is seen indif- 

 ferent individuals and even in different areas of the same individ- 

 ual (see tables, pages 165 to 170). Melonites giganteits, Jackson, has 12 

 columns of ambulacral and 11 of interambulacral plates (plate 4, figure 

 19, and plate 5, figures 21-24}. The next species, 31. etheridf/li, W. Keep- 

 ing (21), from an estimate made by that author, has 12 or 14 columns of 



* Lepidocentrus and Lepidechinus each in one species have 11 columns of interannbulacral phites, 

 so that in this single feature they equal the highest species of the Melonitidse. Lepidesthes colletti 

 surpassed any species of Melonites in having 18 or perhaps 20(?) columns of ambulacral plates. 



fThis description is indefinite. It maybe meant to read that the smaller number is at the 

 ventral portion and the higher number at the ambitus, or further dorsally, \vi»ich would be in ac- 

 cordance with the condition seen in the structure of other species. 



