Vol. 5] Pack. — Tertiary Echinoids of California. 



283 



This form differs from Echinarachinus excentricus in the fol- 

 lowing particulars : The outline of the test varies from quadrate- 

 oval to sub-pentagonal, being seldom sub-circular. The transverse 

 diameter is rarely longer than the longitudinal one. The eccen- 

 tricity is usually greater, the ratio which the distance of the 

 apical shield from the posterior margin bears to the distance 

 from the shield to the anterior margin varies from 1 : 1.9 to 1 : 2.9. 

 The arch of the upper surface commences at the margin and not 

 at the ends of the petals; also the margin is somewhat thicker. 

 The lateral petals diverge with a smaller angle, from 80° to 105°. 

 The apex is more eccentric posteriorly. The lateral petals are 

 almost twice the length of the posterior ones, which latter are 

 often almost circular in outline. 



Occurrence: Lower Pliocene or upper Miocene; Capitola, 

 San Gregorio, Coalinga, Kettleman Hills, Buena Vista Lake. 



The separation of a form Ecliinaradiinus ashleyi, based upon 

 the sub-pentagonal outline of the test, seems hardly justified. 

 This variety was suggested by Professor Merriam after the 

 examination of a small amount of material from Capitola. Since 

 then a large number of specimens of Echinarachinus gibbsii were 

 collected by him at a single locality in the Kettleman Hills, from 

 a bed but four feet in thickness. In this collection specimens 

 were found with almost any outline from quadrate-oval to sub- 

 pentagonal, the transverse diameter sometimes being the greater 

 one. 



Issued July 21, 1909. 



