258 ROBERT TRACY JACKSON ON ECHINI. 



With present knowledge it is impracticable to make a key for distinguishing each of the 

 numerous and imperfectly known species of Archaeocidaris. An attempt has been made, 

 however, to give a key that will distinguish groups of species which have similar characters 

 as regards the structure of the primary spines, and these will serve as an aid in identifying any 

 case in hand in which the spines are in place. 



Key to the Groups of Species of Archaeocidaris.^ 



Spines circular in section, smooth, without spinules or nodose elevations: 

 A. worthnii Hall (America), p. 259. A. hvujiKpina Newberry (America), p. 261. 



A. leqrandcnsis Miller and Gurley (America), A. (jJuhrispina (Phillips) (Europe), p. 2G1. 



p. 2G0. 



Spines circular or rarely elliptical in section, with numerous short spinules which are nearly 

 uniform in size and are directed distally : 



A. nrrei (Miinster) (Europe), p. 262. A. aculeata Shumard and Swallow (America), 



A. rossica (Buch) (Europe), p. 263. p. 268. 



A. agassizi Hall (America), p. 266. A. shumardana Hall (America), p. 268. 



A. iUinoiscnsis Worthen and Miller (America), A. edgarcnsis Worthen and Miller (.America), 



p. 266. p. 269. 



A. coloradensis nom. nov. (America), p. 267. A. newberri/i Hambach (America), p. 269. 



A. kcokuk Hall (America), p. 267. A. frudifer White (America), p. 269. 



A. gracilis Newberry (America), p. 267. 



Spines circular in section, with large thorn-like spinules directed distally: 

 A. norwoodi Hall (America), p. 270. A. cratis White (America), p. 272. 



A. parado.ra (Eichwald) (Europe), p. 270. A. acanthifcra Trautschold (Europe), p. 272. 



A. mucroruda Meek and Worthen (America), 'A. pizzulana Gortani (Europe), p. 272. 



p. 271. A. megastyla Shunmrd and Swallow (America), 



A. dininnii White (America), p. 271. p. 273. 



Spines with two lateral wing-like expansions: 

 .1. Jiiangvlata Shumard and Swallow (America), p. 273. 



Spines densely covered with long spinules directed distally: 

 A. ornata (Eichwald) (Europe), p. 274. A. ouraycnsis Girty (America), p. 274. 



Spines triangular in section, with spinules on the angles and directed distally: 

 A. triplex White (America), p. 274. A. triscrrata Meek (America), p. 275. 



A. triserialis (M'Coy) (Europe), p. 275. 



' A number of species of Archaeocidaris, or of species referable to that genus, which are not known well enough to 

 intercalate in the systematic series are taken up later. These are: Archaeocidaris konincki, ladina, scolica, selwyni, sixi and 

 trautscholdi, Cidarites tennesseae, Echinocrinus anceps, spinosus and siriatus, considered under Incertae Sedis. Also 

 Archaeocidaris tirolensis, Echinocrinus cidnriforniis and pomum considered under Nomina Nuda. 



