310 ROBERT TRACY JACKSON ON ECHINI. 



It has proved unfortunate as Baily's figure has been extensively copied as a typical apical disc 

 for the Palaeozoic. The genitals are high and wide; E and G have each three pores, but C 

 apparently has four pores and A has five genital pores as seen in Plate 29, figs. 3, 6. By over- 

 sight, in Plate 31, figs. 1 and 4, only three instead of five pores are shown in genital A. A 

 number of periproctal plates are in place, they are polygonal in outline, and doubtless filled the 

 area, as shown by Baily in his reconstruction (pp. 89, 173). The second specimen on this slab 

 is much less complete, but shows two columns of ambulacral plates in an area with pore-pairs 

 uniserial, also five columns of plates in one interambulacral area. 



In the specimen in the Museum of Practical Geology (Plate 30, fig. 11; Plate 31, fig. 5) 

 there are two columns of plates in each ambulacral area with pore-pairs superposed, and five 

 columns of plates in an interambulacral area. A number of spines are in place, as described 

 above, and these are of interest as the only ones yet known in the genus. 



The specimen from Dinant, Belgium, figured by Fraipont, is of about the same size as the 

 type, and has five columns of plates in each interambulacral area. 



*Palaeechinus (?) minor sp. nov. 

 Plate 29, figs. 7, 8; Plate 31, fig. 6; Plate 32, fig. 7. 



The specimens of this species are all internal sandstone molds, so that surface characters 

 are unknown. Test nearly or quite spherical. The holotype (Plate 29, fig. 7) measures 36 mm., 

 in height, and 35 mm. in diameter, but it is somewhat distorted. The Museum of Comparative 

 Zoology's specimen (Plate 29, fig. 8) is perfect in form and measures 37 mm. both in height 

 and diameter through the mid-zone. It is one of the very few Echini which have been found 

 to be actually spherical. 



The ambulacra are narrow with two columns of low plates in each area. About five am- 

 bulacral plates equal the height of an adambulacral ; pore-pairs are uniserial. While this species 

 is apparently a Palaeechinus, it must be remembered, as I have shown (p. GO), that in Maccoya 

 the plates on the proximal side are all primary in character and pore-pairs are uniserial, while 

 on the external or distal side of the same plates the character is primary, and occluded plates 

 and pore-pairs are biserial (Plate 33, figs. 4, 5, 7, 8). As the views of P. minor are all molds 

 of the interior, it may therefore later prove that the species is really referable to Maccoya, 

 but whichever the genus, the interambulacral features entitle it to specific separation from 

 other species of Palaeechinus and from known American forms of Maccoya. 



In the holotype there are six columns of interambulacral plates in an area (Plate 31, fig. 6). 

 The figure (partially restored ventrally as shown by dotted lines) is made up from two areas. 

 Dorsally columns 6 and 3 drop out, indicating senescence, and a full grown specimen. The 

 oculars are all insert, and genitals are high, with three pores each. 



