CAEBOmFEROUS SYSTEM. 221 



Associated with them, and probably belonging to the same 

 species, are found long, slender, hollow, curved spines. 



Articulates are represented by one species of the trilobite 

 genus Phillipsia, and two ostracoid genera Cy there and 

 Beyrichia. The latter is so abundant at one locality, a 

 couple of miles northward from Pella, that a bed of limestone 

 four feet in thickness is principally composed of its cast-off 

 shells. Specimens of both the other mentioned genera of 

 Articulates are rare. 



Mollusks. The fossil shells of mollusks distinguish this 

 formation more than the remains of any other branch of the 

 animal kingdom. They include the classes Gasteropoda, 

 Lamellibranchiata, Brachiopoda, and Polyzoa. ]STo Cepha- 

 lopoda have yet been recognized in these rocks in Iowa, 

 although they are well known to exist in the formation 

 elsewhere. Gasteropods are not abundant; Lojnellibran- 

 chiates prevail most in some of the layers of the lower 

 division; Brachiopods are more characteristic of the upper 

 division, in the marly clay and shales of which they are 

 sometimes abundant. Polyzoans are not abundant in the 

 strata of Iowa, but at Warsaw, Illinois, the lowest division 

 there contains them in the greatest profusion. 



Radiates are comparatively rare, with perhaps the excep- 

 tion of the large conspicuous coral Lithostrotion, which is 

 frequently met with in southeastern Iowa. Other genera are 

 Zaphrentis, Syringopora, C7icetetes, etc. Crinoids and all other 

 Echinoderms are rare, showing a marked contrast in that 

 respect between this formation and the two which preceded 

 it. Pentremites Koninckana, P. conoideus, Rhynchonella 

 Ottumwa, supposed formerly to be characteristic of the 

 "Warsaw limestone," are all found in the St. Louis limestone 

 of Iowa, but the two former are very rare here. The following 

 species are regarded as more characteristic of the formation 

 than any others: Spirifer Keokuk var. (Hall) Rhynchonella 

 Ottumwa, (White), Athyris ambigua, (Sowerby), and Litho- 

 strotion canadeuse, (Castelnau). The three first named species 

 are as common at Fort Dodge as they are in the southeastern 



