LEPIDECHINUS. 



This genus is considered the lowest member of the family, and is very near to Palaeechinu-. 

 which is also considered the lowest member of its family. Lepidechirms differs from Palac- 

 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, us in Palaeechinus. 

 Hall in describing Lepidechinus, and Keeping in describing Rhoechinus, pointed out the <-lo-e 

 relations of these genera to Palaeechinus. Unfortunately Professor Mull did not (inure f,epid- 

 echinus imbricatus, the type, but did figure what he described a- /.. w/.v/;//;//*. which, how- 

 ever, has very different characters and is here referred to Hyattcrhimis (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, other- 

 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 Hyatle- 

 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 peristmnal) border of 

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

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



Key to tin- N/wnV.v of I,r/i//lr<'hiiuix. 



Four columns of plates in an interambulacral area at the mid-zone . /.. irri-giilaris (Keeping), p. 



Five columns of plates in an interambulacral area at the mid-y.one /.. KHMMU sp. nov., p. 397. 



Six columns of plates in an interamhulacral area at the mid-y.om- . /,. tr.iscllatus sp. nov., p. 397. 



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



'This reference is incorrect as Hall's figure of I.epidcrtiitms, which is /,. [lli/,illirhinu.i\ rnrw/nniw wa* not published 

 in the Geology of Iowa, but in 1868 (and 1870, revised edition) in the Twentieth Report \. V. Stntc Cabinet N tt lit-- 

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



