296 ROBERT TRACY JACKSON ON ECHINI. 



The test is relatively high, though far from spheroidal, and strongly pentagonal in hori- 

 zontal outline, the apices of the pentagon coinciding with the ambulacral areas. It is the most 

 markedly pentagonal of any known regular sea-urchin. The diameter in different specimens 

 varies from 45 to 60 mm. The ambulacrum is petaloid ventrally, narrow dorsally, 6 mm. wide 

 at the widest part of the petal, which is twice that of the same area at the mid-zone. Ambula- 

 cral plates are wide ventrally, narrower dorsally, and ventrally somewhat higher than they are 

 dorsally. Pore-pairs are uniserial, in peripodia, with ventrally the outer pore set somewhat 

 higher than the inner pore of each pair. Interambulacra are very wide, with fourteen columns 

 of plates in each area, the highest number yet known in Echini. The ventral surface is known 

 in only one specimen (Plate 24, fig. 4; Plate 25, fig. 1), but this is very well preserved. It 

 shows that the primordial interambulacral plate is in place in the basicoronal row in each area, 

 two plates in the second row, and three in the third. In area A the fourth column appears in 

 the fourth row, but in areas C, G, and I both columns 4 and 5 appear in the fourth row; and in 

 area E, apparently (partially restored) columns 4, 5, and 6 appear in the fourth row. This is 

 a degree of acceleration of development much ahead of that known in any other type, and may 

 be correlated with the fact that the species attains more columns than are known in any other 

 type. Columns 6 to 11 come in with great rapidity, as shown in the figure, one or more being 

 added in succeeding rows as built. Turning to the dorsal view (Plate 24, figs. 1-3; Plate 25, 

 figs. 3, 4), we see that the columns 13 and 14 come in later and with considerable intervals in 

 the point of introduction, for, as usual in Hyatt's law of acceleration of development, its effect 

 is most marked on the earlier added parts, the later differentiated structures not yet having 

 come within its control. Dorsally, in this remarkable type, we find the dropping out of columns 

 marking reversionary or senescent characters; this is shown in Plate 25, figs. 3, 4. The adam- 

 bulacral columns 1 and 2 drop out, so that 3 and 4 become in further growth adjacent to the 

 ambulacra; also some additional columns drop out, as 10 and 13 in figure 4. The exterior of 

 the plates is marked by numerous closely set tubercles; but I did not distinguish any division 

 into primary and secondary; rather all seem to be of the second class. The peristome is small, 

 and as in H. rarispinus, is plated with ten columns of ambulacral plates only, which appears 

 to be a character of the whole family. Small insert oculars cover the ambulacral areas and 

 laterally in part the interambulacra on either side. Genitals are relatively large and high; no 

 genital pores were recognizable in the sandstone impression. Impressions of a few small angular 

 plates in the periproct show that it is of the usual Palaeozoic character. The lantern and in- 

 ternal characters of the plates are not preserved in known specimens of this species. 



Waverly Group, Lower Carboniferous, Meadville, Pennsylvania, slab with the holotype 

 in Alleghany College Collection; paratypes in Museum of Comparative Zoology Collection 

 nos. 3,107 and 3,108. 



