SYSTEMATIC CLASSIFICATION OF ECHINI. 225 



are uniserial. The interambulacra have seven or eight columns of strongly imbricate polyg- 

 onal plates in an area. As high ambulacral plates are a primitive character, Koninckocidaris 

 is considered a lower genus than Lepidocentrus which has low ambulacral plates (p. 284). 



In Koninckocidaris cotteaui as described, two ambulacral plates equal the height of an 

 interambulacral ; there are seven columns of interambulacral plates in an area, the median 

 column being the narrowest; spines and tubercles are of two sizes. This, the type of the 

 genus, is from the Lower Carboniferous of Belgium (p. 285). 



Koninckocidaris silurica sp. nov. (Plate 19, fig. 1; Plate 20, figs. 5, 6) has three ambulacral 

 plates equaling the height of one adambulacral. There are eight columns of rhombic inter- 

 ambulacral plates in an area, the plates of column 7 in each area being very narrow in the type. 

 This species is from the Niagara Group, of Rochester, New York, and is of interest as by far 

 the oldest echinoid yet found in America and also the oldest known in the family (p. 285). 



Lepidocentrus is characterized by relatively high spheroidal tests, ambulacral areas narrow 

 throughout, ambulacral plates low, interambulacrals with small eccentric primary and second- 

 ary tubercles. The species are from the Devonian or Lower Carboniferous, and the best 

 known is L. mulleri (Plate 19, figs. 2-5; Plate 20, figs. 8-13), (p. 286). 



Hyattechinus gen. nov. — This genus includes some of the most striking of known Echini. 

 The test is depressed or flattened. Horizontally the outline is circular, pentagonal, or clypeas- 

 triform. The most distinctive character is that the ambulacra are broad, almost petaloid 

 ventrally, narrow dorsally, ambulacral plates are low, pore-pairs uniserial. Interambulacra 

 have from eleven to fourteen columns of plates in each area and a very accelerated development 

 so that new columns are added early and rapidly during development. There are small primary 

 and secondary tubercles on the interambulacral plates. As the species of Hyattechinus present 

 very exceptional characters, these may be briefly stated. In Hyattechinus rarispinus (Hall) 

 (Plate 21, fig. 6; Plates 22, 23) the test is nearly circular, and flattened probably as much 

 as in Recent Phormosoma; ambulacra ventrally are twice as broad as dorsally, with ventral 

 internal spurs as in beecheri. Interambulacra have eleven to thirteen columns of plates in 

 each area, varying within these limits in different areas of the same specimen (Plate 23, fig. 

 3). This species was described as a Lepidechinus by Hall, but is very different from that 

 genus. In Hxjattechinus fentagonus sp. nov. (Plate 24, figs. 1-4; Plate 25, figs. 1-4) the test is 

 compressed, strongly pentagonal, the most so of any known regular echinoid. Ambulacra are 

 ventrally twice as broad as dorsally. Interambulacra have fourteen columns of plates in each 

 area, the highest number known in any echinoid. Dorsally, the adambulacral and some addi- 

 tional columns drop out and therefore fail to reach the apical disc, an indication of senescence. 

 In the remarkable Hyattechinus beecheri sp. nov. (Plate 24, figs. 5-8; Plate 25, fig. 5; Plate 26) 

 the test is depressed with a flat base, moderately pentagonal and elongate, and bilaterally 

 symmetrical through the axis of an ambulacrum and opposite interambulacrum which I take 



