systematic: classification of echini. 229 



be found next the ocular (Plate 56, fig. 6), and passing ventrally, a progressively increasing 

 number of plates occurs until the full species character is attained. This is shown in outline 

 in text-fig. 237. The interambulacra at or near the mid-zone have from three to eleven columns 

 of plates in each area varying with the species, and somewhat in the species, or indeed in differ- 

 ent areas of the same specimen. As in other genera where there is a difference in this respect, 

 the ambulacrum is considered the more essential part and of first consideration in classifica- 

 tion ; the species are therefore erected first on the basis of specialization of the ambulacrum 

 and secondarily on that of the interambulaci'um. Other characters are for the most part 

 essentially similar in all species of the genus. The best known species is Melonechinus multi- 

 porus (Plate 54, fig. 5; Plate 55; Plate 56, figs. 2-13; Plate 57; text-figs. 237, 245, 

 246, p. 382; Plate 60, figs. 1, 2), (p. 359). 



Having considered briefly the family of the Palaeechinidae, it is seen that the genera, and 

 in atldition in Melonechinus the species, present a progressive series as regards differential 

 evolution of ambulacral characters; also, where a complex ambulacrum is attained as a character 

 gathered from the mid-zone, the ventral plates built in the youth of the individual are simpler. 

 The young last added dorsal plates as a localized stage in development also present a simpler 

 condition. This relation is expressed diagrammatically in text-fig. 237. This is not a schematic 

 diagram, as each of the several figures is taken directly from the Plates as indicated. 



In the figure the species characters are given in the middle horizontal row representing 

 the mid-zone. In the lowest row are given the characters taken at or close to the ventral 

 border of the corona ; in the uppermost row are given the characters of plates at or close to the 

 ocular plates. Between the lowest row or the uppermost row and the mid-zone are given 

 the intermediate characters, both ventral and dorsal. 



In Palaeechinus elegans and other species of the genus the plates are simple primaries 

 throughout the length of the ambulacrum, the simplest condition in the family. The plates 

 are wanting at the ventral border in the known specimens of this species, therefore in the diagram 

 a gap occurs at this point. In Maccoya biirlingtonensis the plates ventrally are simple pri- 

 maries, at the mid-zone primaries and occluded plates alternating, and dorsally simple primaries. 

 In Lovenechinus missouriensis ventrally the plates are simple primaries, higher up alternately 

 primaries and occluded, and at the mid-zone demi- and occluded plates. Passing dorsally, 

 we find primary and occluded plates alternating, and close to the dorsal border primaries only. 



In Oligoporus danae again, the plates ventrally are simple primaries, above that alternately 

 primaries and occluded, above that demi- and occluded plates. At the mid-zone there are in 

 addition scattered isolated plates which is the generic feature. Passing dorsally we find 

 demi- and occluded plates only, which doubtless were succeeded near the oculars by primaries 

 only, but such are not preserved in available specimens. 



In Melonechinus we have the most specialized genus with from six to twelve columns of 



