LEPIDESTHES. 415 



specimens a flow of anal plates into the interambulacrum, similar to the flow of the ambulacra! 

 plates of the corona onto the buccal plates of the actinal system." This view of an entirely 

 distinct origin for interambulacral plates from what obtains in all other Echini I believe i- M 

 mistake (pp. 63, 110, 287). Mr. Agassiz (1881, p. 95; 1904, pp.96, 104) also says that II.I.T- 

 ambulacral plates in the Echinothuriidae and apparently in the IVrisrhoechinoida are formed 

 by the splitting up of earlier plates. I believe this view is incorrect as discussed (pp. 64, 362). 

 Yet it is possible that coronal plates may originate by splitting, as a peculiar case of the split- 

 ting of ambulacral plates is shown by Dr. Mortensen (1907, text-fig. 27, p. 171) in Sperosoma 

 grimaldi Koehler. This case was overlooked in my discussion of the origin of plates by split- 

 ting (p. 28). In Sperosoma the primary element of each compound ambulacral plate in painu 

 adorally in development becomes divided by a vertical secondary suture, so that as a n-Milt 

 there exist two admedian columns of imperforate ambulacral plates. I know of no other similar 

 case in coronal plates, and Messrs. Agassi/ and Clark (1909, p. 194) say of this genus. " Tin- 

 existence of a pair of median columns of imperforate ambulacral plates is a feature in which 

 Sperosoma is absolutely unique among all recent regular Echini." I believe the case of 

 Sperosoma and the splitting of genitals and rarely of oculars as discussed (pp. 28, 168-170), 

 are the only cases in Echini in which plates have been shown to be derived by the splitting of 

 preexistent plates. 



The type species is Lepidesthes coreyi Meek and Worthen, from the Lower Carboniferous 

 of America. This genus is striking on account of the great development of the ambulacra (com- 

 pare text-figs. 2-14, p. 54) and the relative reduction of the interambulacra. In ambulacral 

 differentiation it is surpassed only by the new genus Meekechinus. The geologically nldr-t 

 species of Lepidesthes occurs in the Devonian of England, and the geologically highest, als<> 

 the structurally highest species, is found in the Coal Measures of North America. 



Key to the Species of Ijepidesthes. 



Eight columns of plates in each ambulacral area and three columns of plates in each intcrambiilacral arva 

 at the mid-zone . . . / n-nrth,-ni 'Jackson, p. -41(1. 



Probably eight or more columns of plates in an ambulacral area, with four known columns of plate.-, in 

 an interambulacral area . . . . / '<" ''* Trautsclmld, p. II V 



Eight columns of plates in each ambulacral area and five columns of plates in each interanibulaeral area 



L. formosa Miller, p. 41 v 



(?) Incompletely known, apparently eight columns of plates in an ambulacral area and seven columns of 

 plates in an interambulacral area .... /< drroninnix AMiidborne. p. IL'O. 



Ten or more columns of plates in an ambulacral area and five columns of plates in an iiiterambulacral area 



/,. spcctiibili.t (\Vorthen and Miller), p. 421. 



Ten or twelve columns of plates in an ambulacral area and six columns of plates in cadi interambulacral 

 area '. . . . /.. run-ill Meek and \Vortlicn, |>. I.':'.. 



