50 ROBERT TRACY JACKSON ON ECHINI. 



Bateson (1894) reports two cases of "total" hexamerous Echini. One is a Galerites, in 

 which there are six ambulacra and interambulacra as seen from below; the dorsal portion and 

 jaws are not shown. I have-not seen Meyer's original paper where this was described. The 

 other is a six-rayed Amblypneustes, but there is no description of the specimen. Bateson also 

 records three specimens in which there is an accessory ambulacrum. These, with de Loriol's 

 and Ribaucourt's cases above mentioned and the eight more or less completelj' hexamerous 

 specimens here described, include all cases I know of in which there is an increase in parts 

 over the typical pentamerous system in Echini. It is evidently a very rare variation. 



Other variations from the normal have a certain interest. Four plates in the periproct 

 is typical of Arbacia pundulata, but as later shown, variations of fewer or more than this number 

 are common (text-figs. 200-205, p. 175). Variations in the number of genital pores are common, 

 also as regards the distribution of madreporic pores. The variants of genital plates, or genitals 

 and oculars, are all discussed later. 



The above variations from the pentamerous symmetry can all be considered as monstrosi- 

 ties. Variation of another kind is more common, and interesting from another point of view; 

 that is, variation in which the whole animal, or a part, is more or less fully developed in the 

 direct line of the differential development or evolution of the species. This in Palaeozoic 

 Echini is marked as radial variation, which was discussed in my earlier paper (1896, p. 151), 

 and is shown in many cases here. An example is seen in Lovenechinus missouriensis, which 

 has five columns of interambulacral plates with often a sixth column represented by one or 

 two plates near the mid-zone. In Plate 41, fig. 1, the sixth column is represented in areas 

 E, G, I, but in areas A, C, there are only five columns. In the young specimen (Plate 40, 

 fig. 1) there are five columns in four areas, but in area E the fifth column is represented by only 

 a single plate, a great departure from the typical and a close approach to the character seen in 

 lower species of the genus, as L. lacazei (Plate 36, figs. 1-6), which has only four columns of 

 interambulacral plates in an area as a specific character. 



As shown in my earlier paper in detailed studies of Melonechinus muUiporus, a given 

 column of interambulacral plates, especially the columns from 5 upward, may come in at vary- 

 ing zones in different areas of the same specimen, showing considerable variation in the rate of 

 radial development. This is shown well in the zone of introduction of the seventh column 

 in Lovenechinus septies (Plate 45, fig. 1). The actual number of columns, or differential char- 

 acter, may vary in ilifferent areas of the same specimen, as in Hyattechinus rarispinus (Plate 23, 

 fig. 3). In Melonechinus muUiporus (Plate 54, fig. 5) there are nine columns in area .\, but only 

 eight in the four other areas; in Plate 60 the specimen figure 2 has nine columns in all areas, 

 but figure 1 has eight in all areas; Plate 55, fig. 3, has nine columns in areas C, E, and G, 

 but the other areas are not known definitely; the specimen, Plate 55, figs. 1, 2, and Plate 57, 

 has only eight columns in all five areas. A specimen may have but seven columns of inter- 



