DESCRIPTIONS OF THE SPECIES. 167 



of this type which not only has the alternating covering-plates of the ambulacra 

 preserved, but also shows traces of the domo covering the peristomial area and the 

 spiracles (PI. I. rig. 8). This structure has been described both by Shumard and by 

 Hambach, whose observations have been already discussed on pp. 69, 70. 

 Locality and Horizon. Chester, Illinois: Chester group, Subcarboniferous. 



Pentremites obbsus, Lyon . 



Pentremites obesus, Lyon, in Owen's 3rd Report Geol. Survey Kentucky, 1857, p. 169, 

 t. 2. f. 1 a-d. 



Remarks. A single large and imperfectly preserved specimen probably belongs to 

 this species. It possesses a rather more slender form than that figured by Lyon, and 

 the ambulacra are straighter. The plates are very thick, and the radio-deltoid sutures 

 have the sigmoidal outline of those of P. obesus. There are over ninety-five pores' on 

 each side of an ambulacrum, but Lyon describes as many as one hundred and fifty. 



Locality and Horizon. Unknown. 



Pentremites pyriformis, Say. 



(PL I. figs. C, 7 ; PI. II. figs. 21-30; PI. XII. figs. 13, 15 ; PI. XVIII. fig. 3.) 



Pentremites pyriformis, Say, Journ. Acad. Nat. Sci. Pbilad. 1823, vol. iv. pt. 2, p. 294. 



Pentremites pyriformis , Say, Zool. Journ. 1825, vol. ii. no. 7, p. 311. 



Pentremites pyriformis, Troost, Trans. Geol. Soc. Pennsylv. 1835, vol. i. pt. 2, p. 229, 



t. 10. f. 8. 

 Pentremites pyriformis, Owen, American Journ. Sci. 1842, vol. xliii. p. 20, f. 3. 

 Pentatrematites pyriformis, Roemer, Arckiv f. Naturgesck. 1831, Jakrg. xvii. Bd. i. p. 354, 



t. 5. f . 9 a-c. 

 Pentremites pyriformis, Hall, Iowa Geol. Report, 1858, vol. i. pt. 2, p. 693, t. 25. f. 16. 

 Pentremites symmetricus, Hall, toe. cit. p. 691, t. 25. f. 14. 

 Pentremites pyriformis, Billings, Geol. Survey Canada Reports, Decade iv. 1859, p. 20, 



f. 5 .V 7. 

 Pentremites pyriformis, Dujardin & Hupe, Hist. Nat. Zooph. Eckinod. 1862, p. 91. 

 Pentremites pyriformis , Shumard, Trans. St. Louis Acad. Sci. 1865, vol. ii. no. 2, p. 385. 

 Pentremites pyriformis , Billings, American Journ. Sci. 1870, vol. 1. p. 228. 

 Pentremites pyriformis, Wachsmnth & Springer, Proc. Acad. Nat. Sei. Philad. 1879, 



t. 17. f. 5. 



Sj). Char. Calyx pyriform, tapering to both extremities ; summit truncate or some- 

 what convex, and more or less narrowed ; base small and narrow ; section penta- 

 gonal, with the sides quite flat ; periphery at the radial lips, which vary in position. 

 Basal plates forming a small cup, which is often rendered more prominent below by 

 the adherence of the top stem-joint. Eadial plates oblong, convex, lateral margins 

 6ubparallel, slightly spreading ; bodies of variable length, cariuate in the middle line ; 



