PERISCHODOMUS. 403 



The peristome is known only fragmentarily, but some ambulacral plates extend on to 

 this area below the primordial interambulacral plates (Plate 62, fig. 6), indicating' that there 

 are ambulacral plates only on the peristome, as in Lepidesthes (Plate 68, fig. 3). 



The apical disc is small relatively to the diameter of the test, but I have no measure- 

 ments. Of the apical disc in one specimen one small imperforate ocular is in place, situated 

 between the genitals (Plate 64, fig. 8). The genitals are relatively low and wide, with from 

 five to seven or eight pores each. The lantern is known only from pyramids, which are stout 

 and practically like those of Lepidesthes (Plate 68, figs. 9-12). 



Lower Carboniferous Limestone, Hook Head, County Wexford, Ireland, cotype, Sedgwick 

 Museum, Cambridge, England; also a second specimen in same museum collection no. 16; 

 coty^pe. Science and Arts Museum Collection, Dublin; Museum of Comparative Zoology 

 Collection 3,071 ; probably the same locality, two specimens, Trinity College, Dublin, Museum 

 Collection; Yorkshire, England, three specimens, British Museum Collection E 10,686 to 10,688; 

 Coplaw, Chtheroe, Lancashire, Museum of Practical Geology 16,310, 16,311; and Ulverstone, 

 Lancashire, the same collection 16,314; Hook Head, Ireland, Museum of Practical Geology 

 16,315; Clitheroe, Lancashire, Mr. H. L. Hawkins's collection. 



The structural characters of the species were gathered from various specimens, and I 

 will now consider these with any additional features which they show. The specimen in the 

 Dublin Science and Arts Museum, from the Griffith Collection, which is doubtless M'Coy's 

 second specimen, is the dorsal half of a test (Plate 29, fig. 5). It occurs on a slab from Hook 

 Head, with two specimens of Palaeechinus elegans, one of which is the original of M'Coy's 

 (1844) and Baily's (1865b) figures of that species as earlier described (p. 309). Baily (1865b, 

 p. 63) says of this slab that there are three specimens of Palaeechinus elegans, but one of the 

 three is evidently this Perischodomus. The specimen is somewhat crushed, and the ambulacral 

 areas are largely hidden by the over-riding of the adjacent adradial plates; but some ambul- 

 acral plates are seen. There are five columns of interambulacral plates in each area, the plates 

 showing thin lateral and aboral imbricating edges, also a number of eccentric primary tubercles. 



The specimen from Hook Head, described by Keeping, which is also one of M'Coj^'s origi- 

 nal specimens, is one of the best specimens of the species known. This choice specimen (Plate 

 64, figs. 6-8) is in the Sedgwick Museum, Cambridge, England, where I studied it through the 

 kindness of Professor Woods. At the mid-zone, ambulacral plates are alternately somewhat 

 narrowed, but none was seen actually cut off from contact with the interambulacra. Pore- 

 pairs at the mid-zone are slightly biserial. Some of the ambulacral plates show well the lateral 

 bevel (Plate 64, fig. 6). The interambulacra have the typical species character of five columns 

 of plates in each area, the plates bear primary and secondary tubercles (Plate 64, fig. 8), and 

 two of the columns drop out passing dorsally. The apical disc is the best known in the species. 

 There are one small imperforate ocular and four genitals in place. The genitals have from five 



