290 PROCEEDINGS OF THE ACADEMY OF [1889. 



Platyceras latum Keyes. 



Platyceras latum Keyes, 1888. Proc. Am. Philos. Soc, vol. 

 XXV, p. 242. 



A broad depressed form from the upper Burlington limestone. 

 In this shell there are no folds or imbricating lines of growth 

 observable; and it is very probable that the habits of this speeies 

 differed somewhat from its nearest relative P. equilaterum. It is of 

 rare occurrence, though associated forms are quite common. 



Platyceras obliquum Keyes. 



Platyceras obliquum Keyes, 1888. Proc. Am. Philosophical Sue., 

 vol. XXV, p. 241. 



A rather large robust species, which, like the majority of the 

 raolluscan shells from the white limestones of the upper Burlington 

 division, is usually exfoliated ; and crumbling quickly away, it leaves 

 only the internal easts. 



Platyceras quincyense MeChesney. 



Platyceras quincyense MeChesney, 1861. Desc. New Foss., p. 

 90. 



Platyceras ijuiucyense Meek and Worthen, 1868. Geo!. Sur. 

 Illinois, vol. Ill, p. 510. 



Not uncommon in the upper Burlington; and sometimes found 

 resting on the vault of Physetocrhuis ventricosus (Hall). One of the 

 most important distinctive features assigned to this species is its 

 peculiar quinquelobate appearance ; but the real cause of the five 

 broadly rounded lobes did not suggest itself until the discovery of an 

 individual adhering to the dome of a erinoid in which the inter- 

 radial areas were considerably depressed, leaving the ambulacra 

 rather highly elevated. The growing margin of the gasteropod 

 shell, in following the inequalities of the surface upon which it rested, 

 gradually assumed the lobate form. 



Platyceras tribulosum White. 



Platyceras tribulosum White, 1883. 12 th. Ann. Rept. U. S. Geol. 

 Sur. Ter., pt. i, p. 168. 



First known from this locality, where it is found in the upper beds 

 of the Burlington limestone. It is one of the few spiniferous species 

 from the American paleozoic rocks ; and differs from P. biserale 

 Hall, of the same horizon, chiefly in having three, instead of two, 

 longitudinal rows of spines. These appendages, though seldom 

 preserved entire, are long, slender and tubular. It may be added 



