406 ROBERT TRACY JACKSON ON ECHINI. 



The pyramid is wide-angled with a moderately deep foramen magnum. A half-pyramid seen 

 from within shows the dental slide not quite reaching to the base of the foramen, the usual 

 Palaeozoic character. The pyramids show the suture for the epiphyses, but these structures 

 are wanting. The longest spine measures 12 mm. in length. This specimen is the most instruc- 

 tive for the lantern of any known in this species. The specimen from Hook Head in the 

 Museum of Practical Geology Collection 16,315, is a crushed test seen from the dorsal side. 

 It shows two genital plates which are wide, rather low, and each has five genital pores. 



In the British Museum Collection E 10,686 to 10,688, there are three specimens of this species 

 from Yorkshire. The specimens consist of small slabs with interambulacral plates bearing 

 eccentric primary and secondary tubercles. There are fragments of spines, but no ambulacral 

 plates; one of the specimens, E 10,686, has two half-pyramids similar to those of Lepidesthes. 

 One of these as seen from side view shows ridges for the attachment of interpyramidal muscles. 



Mr. Hawkins's specimen, which he collected at Clitheroe, consists of dissociated interam- 

 bulacral plates, a few ambulacrals, and a very good half-pyramid, which has ridges for the 

 attachment of interpyramidal muscles. 



All of the English specimens seen are clearly referable to the genus. The plates are thicker 

 and more massive than in the Irish specimens, so that they may represent a distinct species. 

 In their very fragmentary condition it is best to leave them in biserialis. 



Perischodomus illinoisensis Worthen and Miller. 

 Text-fig. 247. 



Perischodomus illinoisensis Worthen and Miller, 1883, p. 333, Plate 31, fig. 8; Klein, 1904, p. 20; Lambert 

 and Thiery, 1910, p. 122. 



Perischodomus (? f) illinoisensis Keyes, 1895, p. 191. 

 Tretechinus illinoisensis Tornquist, 1897, p. 784. 



I have not seen specimens of this species, and the characters are 

 taken from the original description. Ambulacra are narrow, with two 

 columns of low, interlocking plates, with a pore-pair in each plate. 



~ n tj 



Ambulacral plates not uniform in size or shape, though most of them 



TEXT-FIG. 247. Peri- , 



schodomm illinoisensis seem to have an imperfectly pentagonal outline. The ambulacral plate 

 Worthen and Miller. i m k r i ca te adorally. The interambulacra have five, or perhaps seven 



Chester Limestone, Lower 



Carboniferous, Bay City, columns of plates in an area. The plates are very irregular in size, 



imbricating aborally and probably from the center outward ; at least the 

 natural size (after Worth- adradial plates imbricate laterally over the ambulacra. The plates bore 



en and Miller, 1883, Plate . , 



31> fig . 8 ). two kinds of spines. The primary tubercle is moderately large, perfo- 



rate. There is only one plate with a primary tubercle seen, and that 



is in one of the median columns. Secondary tubercles on adradial plates and perhaps [prob- 

 ably] on others. 



