306 ECHINODEEMATA— ECHINOZOA phylum iv 



eccentric and irregularly distributed on interamhulacral plates, with secondary spines 

 and tubercles, or the latter only. Devonian to Permian. 



This family presents a wide ränge of characters, and includes species with 

 very specialised features, particularly as regards an extreme development of 

 ambulacral areas. 



Lepidechinus Hall (Ehoechinus Keeping). Two columns of plates in each 

 ambulacral area. Four to eight columns of plates in each interamhulacral 

 area. Plates quite thick, imbricating moderately. Secondary tubercles only. 

 Genital plates as far as known with only one pore each, the only instance 

 known in the Paleozoic. This genus, the lowest of the family, difFers from 

 Palaeechinus which it approaches, principally in the fact that the plates are 

 imbricate. The genus has been misunderstood because Hall referred to 

 it the species rarispinus, which is now referred to Hyattechinus. Lower 

 Carboniferous ; Europe and North America. 



Perischodomus M'Coy (Fig. 366, h). Two columns of plates in each am- 

 bulacral area. Five columns of plates in each interamhulacral area. Plates 

 imbricating strongly. Eccentric perforate primary with secondary tubercles 

 on interamhulacral plates. Genital plates with many pores. The most 

 completely known species is P. biserialis M'Coy, Lower Carboniferous of Great 

 Britain ; a second imperfectly known is P. iUinoisensis Worthen and Miller, 

 Lower Carboniferous ; North America. 



Perischocidaris Neumayr. Six columns of plates in each ambulacral area. 

 Five columns of plates in each interamhulacral area. Plates apparently 

 imbricating moderately. Eccentric primary tubercles on certain adradial 

 plates, with secondary tubercles on the same and usually alone on other 

 interamhulacral plates. Lower Carboniferous ; Ireland. 



Proterocidaris Koninck. Four columns of plates in each ambulacral area. 

 T.welve to thirteen columns of plates in each interamhulacral area. Plates 

 strongly imbricating. Small primary with secondary spines and tubercles on 

 interamhulacral plates. Lower Carboniferous ; Belgium. 



Lepidesthes Meek and Worthen {Eyboechinus Worthen and Miller) (Figs. 

 365, 367, ; 434). Eight to sixteen columns of plates in each ambulacral 

 area. Three to seven columns of plates in each interamhulacral area. Plates 

 are strongly imbricating and are all of uniform size. Secondary spines and 

 tubercles only, Test elliptical, obovate or spherical. This genus has more 

 species and a wider geological ränge than any other of the family. Ambulacral 

 plates are very regulär in form, either rhombic or hexagonal. There may be 

 as many as sixteen columns of ambulacral plates in an area, e.g. L. colletti 

 White, in which species with an extreme ambulacral development there are 

 only four columns of interamhulacral plates (Fig. 434). In one species, 

 L. wortheni Jackson, there are eight columns of ambulacral plates with only 

 three columns of interamhulacral at the mid-zone, but there are four columns 

 ventrally as a youthful stage. Devonian ; Great Britain. Lower Car- 

 boniferous ; Russia, Great Britain, North America. Carboniferous ; North 

 America. 



Pholidocidaris Meek and Worthen (Protocidaris Whidborne). Four to six 

 columns of plates in each ambulacral area. Five to six columns of plates in 

 each interamhulacral area. Plates strongly imbricating. Ambulacral plates 

 large ventrally, small dorsally ; interamhulacral plates dorsally very large in 



