LEPIDOCENTRUS. 287 



ambulacral areas with many (5 to 11) columns of plates which are moderately thin and imbricate 

 strongly aborally and from the center outward and over ambulacrals. Interambulacral plates 

 are rhombic, or more or less hexagonal, and the primordial interambulacral plates are in the 

 basicoronal row. Peristomal plates are unknown in this genus. Mr. Agassiz (1881, p. 79) 

 says that in Lepidocentrus there is no distinct line of division separating the coronal interam- 

 bulacral plates from those belonging to the actinal membrane. He further says that in a 

 remarkably well preserved , specimen from the Lower Burlington Limestone, the ambulacral 

 plates are continued to the very teeth, and the same is the case with the interambulacral plates. 

 Unfortunately Mr. Agassiz did not figure this structure, or mention in what species it occurred. 

 The peristome is not known to me in this genus, but in two other genera of the family, only 

 ambulacral plates exist on the peristome, and such a structure as Mr. Agassiz describes is not 

 known to me in any sea-urchin, fossil or Recent. The genus is not known from the Burlington 

 Limestone excepting as implied by Mr. Agassiz's remarks, and this is the first mention of its 

 occurrence in America. It is on this statement apparently that Duncan (1889a) records the 

 genus from this country. Insert oculars, and genitals with many pores are known in one 

 species, the new L. whitfieldi (Plate 21, fig. 5). The apical disc is proportionately small. 



Mr. Agassiz (1881, p. 80) says of Lepidocentrus that, "the genital plates bear about the 

 same proportion to the plates of the anal system which we find in some of the recent Echino- 

 thuridae, and there exists at the apical extremity of the coronal plates the same difficulty in 

 defining where the interambulacral plates terminate and the plates of the apical system begin." 

 I do not know the basis of Mr. Agassiz's statement, but in this genus, Lepidocentrus, and in 

 all Palaeozoic Echini that I know (excepting possibly Koninckocidaris silurica, p. 286) the geni- 

 tal and ocular plates, when preserved, are in contact throughout, so that the interambulacra 

 are separated from the periproct by the oculo-genital ring, not in contact with it, as may occur 

 in the Echinothuriidae (text-fig. 170, p. 149; p. 414). 



Lepidocentrus occurs in the Devonian and Lower Carboniferous; the type species is 

 L. eifelianus Miiller. 



Key to the Species of Lepidocentnis} 



Fi\e columns of interambulacral plates in an area . . . . L. rhenanus (Beyrich), p. 288. 

 Eight columns of interambulacral plates in an area . . . . L. drydenensis (Vanu.xem), p. 288. 

 Ten or eleven columns of interambulacral plates in an area, the plates nearly rhombic in form 



L. miiUeri Schultze, p. 289. 

 Eleven columns of interambulacral plates in an area, the plates small and rounded hexagonal 



L. whitfieldi sp. no\-., p. 290. 



Number of columns of interambulacral plates in an area unknown, plates similar to those of milUeri, but 



thinner and more rounded in outline i. (7/(//o?n/s Miiller, p. 291. 



' Three species of Lepidocentrus, namely L. desori, duponti, and gandryi, of which no description has been published, 

 are considered under Nomina Xuda. 



