PERISCHODOMUS. 401 



Perischodomus M'Coy. 



Pcrischodomus M'Coy, 1S49, p. 253; 1854, p. 114; Loven, 1874, p. 40; Keeping, 1876, p. 35; (pars) Dun- 

 can, 1889a, p. 10; Jackson, 1896, p. 242; Tornquist, 1897, p. 783; (pars) Klem, 1904, p. IS; (pars) 

 Lambert and Thiery, 1910, p. 122. 



Tretechinus Tornquist, 1897, p. 784. 



Test high, spheroidal. Ambulacra are narrow, composed in each area of two columns 

 of low plates. Interambulacra are wide, with many columns of plates (five in the known species) 

 in each area. The interambulacral plates imbricate strongly aborally and from the center 

 laterally and over the ambulacra on the adradial sutures. The primordial interambulacral 

 plates are in place in the basicoronal row. Adambulacral plates bear medium-sized eccentric 

 perforate primary tubercles with secondary tubercles, and median interambulacral plates bear 

 primary and secondary tubercles, or the latter only. Spines are medium-sized primaries and 

 small secondaries. Plates of the peristome are imperfectly known, but apparently are ambula- 

 cral only. Oculars are imperforate, genitals low and wide, with numerous pores. The lantern 

 is known from pyramids which are similar to those of Lepidesthes and other Palaeozoic Echini. 



The characters of this genus are based on Perischodo7nus biserialis, the type species of the 

 genus and the only one which is well known. This genus differs from Lepidechinus in that 

 the interambulacral plates, in part at least, bear primary with secondary tubercles. It differs 

 from the other genera of the family in that it has two instead of more than two columns of 

 plates in each ambulacral area. In my earlier paper (1896, p. 242) I included Perischodomus 

 in the Lepidocentridae, but with fuller knowledge of Lepidechinus and Perischodomus it seems 

 that both genera belong in the Lepidesthidae as primitive representatives of the family. 



Kcj/ to the Species of Perischodomus .'^ 



Five columns of plates in each interambulacral area, test large . . . P. biserialis M.'Coy, p. 401. 

 Five, or perhaps more, columns of plates in each interambulacral area, test small (imperfectly known). 



P. illinoisensis Worthen and Miller, p. 400. 



*Perischodoinus biserialis M'Coy. 



Text-fig. 30, p. 70; Plate 29, fig. 5; Plate 62, figs. 6, 7; Plate 64, figs. 2-8. 



Perischodomus biserialis M'Coy, 1849, p. 253, text-figs, a-c; 1854, pp. 114, 115, text-figs, a-c; Desor, 

 1858, p. 157; Quenstedt, 1875, p. 374, Plate 75, fig. 13; Loven, 1874, p. 40; Keeping, 1876, p. 36, 

 Plate 3, figs. 1-5; Jackson, 1896, p. 226; Klem, 1904, p. 18; Lambert and Thiery, 1910, p. 122. 



Perischodomus bisserialis Pome], 1883, p. 114. 



Excellent material of this species which I had the opportunity of studying in Dublin, 

 also in London and Cambridge, England, shows the characters of this species well, and is 

 structurally very interesting. M'Coy mentioned two specimens, one in Cambridge, and 



' Perischodomus magnus is considered under Incertae Sedis, p. 453. 



