LEPIDECHINUS. 395 



This genus is considered the lowest member of the family, and is very near to Palaeechinus, 

 which is also considered the lowest member of its family. Lepidechinus differs from Palae- 

 echinus chiefly in the fact that the plates are imbricate and that adambulacral plates bevel 

 over the ambulacrals instead of ambulacrals beveling over the adambulacrals, as in Palaeechinus. 

 Hall in describing Lepidechinus, and Keeping in describing Rhoechinus, pointed out the close 

 relations of these genera to Palaeechinus. Unfortunately Professor Hall did not figure Lepid- 

 echinus imbricatus, the type, but did figure what he described as L. rarispinus, which, how- 

 ever, has very different characters and is here referred to Hyattechinus (p. 292). Naturally 

 the understanding of Lepidechinus has been taken largely from rarispinus with the result that 

 the genus Lepidechinus has been misinterpreted. As I have studied and here figure the type, 

 it is hoped that the relations expressed may prove correct. 



Mr. Agassiz (1874, p. 648) says, "In Hall's genus Lepidechinus the ambulacra and inter- 

 ambulacra lap in opposite directions. Interambulacra in eight rows; outer pentagonal, others 

 hexagonal. Hall, in his Report on the Geology of Iowa, has given us excellent figures of the 

 tubercles, spines, and portions of the ambulacral and interambulacral plates (PI. XXVI)." 

 This reference is quite mistaken, as in the work and Plate referred to. Professor Hall figured 

 Archaeocidaris, not Lepidechinus. Hall never figured Lepidechinus imbricatus, which has 

 eight columns of plates in an interambulacrum, and which is here figured for the first time 

 (Plate 62, fig. 5; Plate 64, fig. 1). Hall, however, did give a figure of Lepidechinus [Hyatte- 

 chinus] rarispinus, which has eleven columns of plates in each interambulacral area (pp. 292, 

 294; Plate 21, fig. 6; Plate 22, fig. 7). Further, Mr. Agassiz (1892, p. 74) says, "Hall's figures 

 of Lepidechinus in the Geology of Iowa [1858], Vol. I., Part 2, Plate IX, fig. 10,' show three or 

 four plates at the_ actinal edge." Hall's Lepidechinus rarispinus is here referred to Hyatte- 

 chinus rarispinus (pp. 292, 294); his figured specimen, and the only one he did figure (my 

 Plate 21, fig. 6; Plate 22, fig. 7), is a dorsal mold, so that Mr. Agassiz is mistaken in .speaking 

 of it as ventral. In rarispinus there is a single plate at the actinal (or peristomal) border of 

 the test (not "three or four plates") as I show in three species of Hyattechinus (p. 292), as 

 well as in the true Lepidechinus (p. 400). 



Key to the Species of Lepidechinus. 



Four columns of plates in an interambulacral area at the mid-zone . . L. irregularis (Keeping), p. 396. 



Five columns of plates in an interambulacral area at the mid-zone . . L. iowensis sp. nov., p. 397. 



Six columns of plates in an interambulacral area at the mid-zone . . L. tessellatus sp. nov., p. 397. 



Eight columns of plates in an interambulacral area at the mid-zone . . L. imbricatus Hall, p. 399. 



' This reference is incorrect as Hall's figure of Lepidechinus, which is L. [Hyaltcchinus\ rarispinus was not, published 

 in the Geology of Iowa, but in 1868 (and 1870, revised edition) in the Twentieth Report N. Y. State Cabinet Nal. Hist., 

 Plate 9, fig. 10 (.see this memoir, p. 292). 



