PERISCHOCIDARIS. 407 



Chester Limestone, Lower Carboniferous, Bay City, Pope County. Illinois. The type 

 as stated by Worthen and Miller is in the Illinois State Collection no. 2,483. This species is 

 imperfectly known. The fact that it has two columns of ambulacra! plate-, and primary and 

 secondary tubercles on interambulacral plates, with other characters, seem- quite sufficient to 

 refer it to Perischodomus. 



PERISCHOCIDARIS Neumayr. 



Arclmroridtiris (pars) Daily, 1874, p. 42; (pars) 1875, p. Ixviii. 



Perischocidaris Neumayr, 1881, p. 174; Lambert and Thie"ry, 1910, p. 126. 



Prosechinus Pomel, 1883, p. 1 13. 



Homotoechm Sollas, 1892, p. 153. 



Perischodomus (pars) Duncan, 1889a, p. 10; (pars) Klein, 1904, p. 19. 



Test spheroidal. Ambulacra broad with six columns of plates at the mid-zone, composed 

 of wide occluded, narrower demi-, and one column of isolated plates in each half-area (Plate 67, 

 fig. 1). The two median columns of occluded plates in the middle line of each area are elevated 

 in rounded ridges similar to those of Melonechinus. Pore-pairs are in sunken valleys, in throe 

 vertical series on either side, and lie in the outer portion of the ambulacral plates in each 

 half-area. 



The interambulacra are wide with in each area five columns of plates at the mid-zone, 

 the plates are high, convex, polygonal, and rounded on the suture lines. Part of the inter- 

 ambulacral plates bear large, elevated, perforate primary tubercles with secondary tubercles, 

 which latter exist on all coronal plates as shown by Baily (1874). 



The oculars are small, insert, and imperforate, ventrally covering the ambulacra and 

 laterally in part the interambulacra on either side. The genitals are high, wide, and peculiar 

 in that most of them bear a large primary tubercle. Genitals have from three to six pores 

 each. One of the genitals does not show any pores (Plate 67, fig. 3), but this is ascribed simply 

 to imperfection of the mold, as the only known specimen is an external sandstone mold. This 

 same imperforate plate is also without a primary tubercle which exists on the four other genitals, 

 but I see no necessary reason for considering it a madreporite as does Sollas. There are no 

 genitals with sixteen pores, as stated by Harte (1865, p. 68) and Neumayr (1881). The ventral 

 half of the test is unknown. 



This genus differs from the Palaeechinidae in having large perforate primary tubercles 

 on interambulacral plates, also in that the plates are probably imbricate. It differs from other 

 genera of the Lepidesthidae in having melon-like ribs in the ambulacra. It is the first 

 genus of the family systematically considered which has more than two columns of plates in 

 an ambulacral area. 



Type and only known species, Perischocidaris harteiana (Baily) from the Lower Car- 

 boniferous of Ireland, 



