224 ROBERT TRACY JACKSON ON ECHINI. 



but without a basal terrace, a feature which, with the perforation, developed later as the plate 

 was pushed ventrally by younger newly added plates. The base of the corona is extensively 

 resorbed so that several rows of plates have evidently been cut away in the advance of the peri- 

 stome (Plate 9, figs. 7, 8). This is the most extensive resorption definitely known in any Echini, 

 though it may have been as much, or more, in Recent regular Echini, where data are not avail- 

 able to measure the amount of resorption (p. 256). 



The species of Ai'chaeocidaris are numerous but are largely described from isolated spines 

 and plates. With available material perhaps the best way to group the species is on the basis 

 of the character of the spines, as is done under the description of species. The smooth-spined 

 forms are considered first, as smoothness is in general a primitive character; next are grouped 

 the species with spines bearing short small spinules, then those with long thorn-like spinules, 

 etc., associating the species on the similarity of characters in these respects (p. 258). Only 

 six species are known with a more or less complete test, and of these the best material occurs 

 in Archaeocidaris wortheni (Plate 8, figs. 5, 6; Plate 9, figs. 6-11) and A. rossica (Plate 10, fig. 

 10; Plates 11, 12). The species of Archaeocidaris occur in the Lower Carboniferous, Carbon- 

 iferous or Coal Measures, and Permian. 



The genus Lepidocidaris is known only from L. squamosa Meek and Worthen (Plate 



16, figs. 1-3; Plate 17). The ambulacral plates are not all alike, but every third plate is larger 

 and more or less completely abuts against two smaller plates of the opposite half-area (Plate 



17, figs. 1, 3, 8). There are six or eight columns of plates in an interambulacral area; these 

 are high, rounded, with a scrobicular area, but no basal terrace. The margin is thickly 

 studded with secondary tubercles. The primary spines are feebly striate longitudinally and 

 very cylindrical (p. 282). 



The Lepidocentridae is a family of Silurian, Devonian, and Lower Carboniferous Echini 

 that have strongly imbricate plates and includes most striking and interesting species. There 

 are but two columns of plates in each ambulacral area, and from five to fourteen columns of 

 plates in each interambulacral area; the latter is the extreme limit of known Echini. The 

 primordial interambulacral plates are in place in the basicoronal row (Plate 23, fig. 1), as ascer- 

 tained in five species. The peristome is covered with many rows of ambulacral plates only 

 (Plate 23, fig. 1). Oculars where known are small, all insert, and the genitals where known, 

 are wide with many (eight or nine) genital pores (Plate 28, fig. 10). The periproct has numerous 

 plates (Plate 25, figs. 3, 5) and an exceptionally perfect lantern is known in one species (Plate 

 27), presenting the typical Palaeozoic character. The spines are small, eccentrically placed 

 primaries, with secondaries, or the latter only (p. 284). 



The genus Koninckocidaris DoUo and Buisseret was based on K. cotieaui Dollo and Buis- 

 seret, which has not been figured. The most important character is that the ambulacral plates 

 are high, two or three equaling the height of an adjacent interambulacral plate; pore-pairs 



