368 bulletin: museum of comparative zoology. 



"the base much depressed or having the form of a pentagonal dish"" 

 (Geol. survey Illinois, 1873, 5, p. 461). The general outline is much 

 too globose, and too wide above the middle. The ambulacra are also 

 too long. 



Peniremites chesterensis (Kaskaskia limestone of Illinois) has twelve 

 side-plates in five millimeters. Moreover the deltoid plates project 

 slightly above the apertures on the summit. It also differs from 

 Pentremites altimarginatus in having long, comparatively narrow 

 ambulacra, which do not taper downwards appreciably but appear to 

 have subparallel sides. 



Pentremites ohesus (Millstone grit of Kentucky, Kaskaskia limestone 

 of Illinois, Kentucky, and Missouri) resembles Pentremites altimargi- 

 natus in shape somewhat, but is a much larger form. Its ambulacra 

 are rather shorter than in the specimens from Old Baldy, and the 

 deltoids also are shorter. 



Peniremites godoni (Subcarbonif erous ; distribution widespread) 

 differs from this form in having a subtruncate base and ambulacra 

 which extend nearly the whole length of the specimen. Moreover 

 Peniremites godoni is stelliform in cross-section and Pentremites alti- 

 marginatus is not. 



Pentremites robustus (Millstone grit of Kentucky) has all the sutures 

 in the base depressed. The ambulacra are six sevenths the length of 

 the specimen, and the sides are subparallel. It has eighteen side- 

 plates in five millimeters. Our specimens are rather fragile, but in 

 Pentremites robustus " all the main pieces composing the body are of 

 remarkable thickness and strength" (Trans. Acad. sci. St. Louis, 

 1860, 1, p. 630). Moreover there is no rim bordering the inter- 

 ambulacral areas. 



Pentremites maccallici Schuchert (Bangor limestone of Georgia) 

 has twelve to thirteen side-plates in five millimeters while Pentremites 

 altimarginatus has fifteen. The former species is very large and 

 stelliform (Proc. U. S. N. M., 1906, 30, p. 759-760). 



The specific name of this form refers to the raised rim between the 

 interambulacral areas and the ambulacra. 



Pentremites bradleyi Meek. 



Pentremites bradleyi Meek, Rept. U. S. geol. surv. Montana, Idaho, Wyoming,, 

 and Utah, 1873, p. 470. Hambach, Trans. Acad. sci. St. Louis, 1903, 13, 

 p. 56, pi. 5, fig. 7. 



