North American index fossils, invertebrates . f irregular, much smaller ambulacral plates; large spines at scat-tered intervals. Distinguished from Lepidechinus by the muchgreater breadth of its ambulacral areas, by its more numerous rowsof ambulacral pieces and pores and by larger size. P. irregularis Meek and Worthen. Mississippic. Marginal interambulacral plates large, elliptical, each bearing acylindrical spine about one inch long; number of columns 5-6;ambulacral plates most irregular in size, increasing in size and<lecreasing in number towards end of area. Keokuk of I
North American index fossils, invertebrates . f irregular, much smaller ambulacral plates; large spines at scat-tered intervals. Distinguished from Lepidechinus by the muchgreater breadth of its ambulacral areas, by its more numerous rowsof ambulacral pieces and pores and by larger size. P. irregularis Meek and Worthen. Mississippic. Marginal interambulacral plates large, elliptical, each bearing acylindrical spine about one inch long; number of columns 5-6;ambulacral plates most irregular in size, increasing in size and<lecreasing in number towards end of area. Keokuk of Illinois. Subclass Euechinoidea Bronn. Order CIDAROIDA Duncan. X. CiDARis Klein. Test spheroidal. Mouth opening central below. Anal opening subcentral above. Ambulacra narrow, undulating, extending from top to bottom and composed of very numerous plates. The pairs of pores are arranged in a single series, the two pores of each pair rather close and separated by a small knob or ridge. Primary tubercles perforated and crenulated. Fig. 1918. Cidaris texana. «, test restored, X K 5 ^ interambulacral plate en-larged ; c, tubercle much enlarged ; d, portion of ambulacral area enlarged. (AfterClark.) 586 NORTH AMERICAN INDEX FOSSILS. 18. C. texana Clark. (Fig. 1918.) Comanchic. Test large. Ambulacral areas with four rows of granules be-tween the pore-bearing zones on the under side, increased to sixat the ambitus and reduced to two at the apex. Pores oval, sepa-rated by transverse ridges which partially envelop the circular, depressed. Washita of Texas. XI. Leiocidaris large, swollen. Differs from Cidaris in having the twopores of each pair distant and united by a groove. Tubercles
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