236 R. T. JACKSON — STUDIES OF PALiEECHINOIDEA. 



and Tiarechinvs. When the third row of interambulacral plates develop, 

 if it still has 2 plates, then the developinf^ young belongs to the order 

 Euechinoida, but if it has 3 plates in this row, then the young Echinoderm 

 is a member of the order Perischoechinoida. During further development 

 of both ambulacra and interambulacra in the Perischoechinoida we get 

 progressively added those features which differentiate the genera and 

 species of the families, as has been traced in detail in preceding pages. 



The wa}^ in which the development of the ambulacrum may be made 

 use of in tracing the morphological and phylogenetic relations of genera 

 is shown under tlie discussion of Oligoporus, where it is shown in the 

 text and b}^ the diagram (figure 1, page 191) that the several genera of the 

 MelonitidtT may be placed in a natural sequence by the characters evinced 

 in the structure of the plates of the ambulacral areas at the ventral border 

 and the ambitus. 



In regard to tlie use of the interambulacral development, its first col- 

 umns built, numbers 1, 2, 3 and 4, are important as structurally diag- 

 nosing the larger divisions of the Echini to which the animal belongs. 

 After the fourth, the addition of other columns is chiefly of value as aid- 

 ing in the proper systematic placing of species in the genus, for species 

 with a liigher numl)er of columns have evidently passed through stages 

 represented by the adults of species with a lower number of columns, 

 whence those species with more columns are further removed in the line 

 of development from the parent stock.* This use of progressive increase 

 in the number of columns is illustrated in the arrangement of species of 

 Melonitidffi and other genera in the table facing page 242. 



In Paleozoic Ecliini the part wliich the peristome has played in its 

 encroachment on the corona b}^ resorption or the absence of this en- 

 croachment is an important character. In embryos of modern Echini, 

 according to Loven, and in the genus Bothriocidaris, the ventral border of 

 the corona is intact. From this condition, which we will call primitive, 

 we get various departures. In the principal series of Paleozoic Echini 

 which advance in the regular line of variation of the group without aber- 

 rant specializations we find a greater or less amount of resorption. These 

 groups are the Melonitidse, which have apparently one row of plates re- 

 sorbed (Melonites, plate 2, figure 2, Oligoporus plate 6, figure 25) ; the 

 Archa}ocidarida3, which liave several rows of plates resorbed (Arch<xoci- 

 darts, plate 8, figures 43 to 45), and Cidarids (plate 8, figure 47), which also 

 have several rows of plates resorbed. In the aberrant groups of Paleozoic 

 Echini which are not in the regular line of variation of the group we get 



* Of course this should not be taken to mean that all species with a smaller number of columns 

 are the ancestors of species with a higher number; they maj"^ or may not have been ; but what it 

 does mean is that these specios are by this means systematically located as more or less advanced 

 in the special line of variation of the genus. 



