1920] Kew: Cretaceous and Cenozoic Echinoidea 



61 



nearly pentagonal shape and thinner margin. In C. deserti the odd 

 anterior petal is the longest, and slightly open, the remainder being 

 of the same length. The longest petals in C. rotundus are the biviiim, 

 and these are nearly closed. 



Geologic horizon. — Lower Division of the Carrizo Creek beds, 

 Pliocene. 



Localities.- — Coyote Mountain and Carrizo Valley, Imperial County, 

 California. Type from Univ. Calif, loe. 2064. 



Family PIBULARIIDAE Gray 

 Genus SISMONDIA Desor 



SISMONDIA (?) ARNOLDI Twitehell 

 Plate 4, figures 2a, 2b 



Astrodapsis sp. indet. Arnold. U. 8. Geol. Surv. Bull. no. 396, 1909, p. 30, 

 pi. 28, figs. 5-5a. 



Astrodapsis sp. indet. Arnold and Anderson, U. 8. Geol. 8urv. Bull. no. 398, 



1910, p. 128, pi. 50, fig. 5. 

 Astrodapsis sp. indet. 8tefanini. Boll. 8oc. geol. ital., vol. 30, 1911, p. 703. 

 Sismondia (?) ariwldi Twitchell. U. 8. Geol. 8urv. Mon., vol. 54, 1915, 



pp. 182-183, pi. 85, figs, la-lb, 



Holotype.—^o. 165538 U. S. Nat. Mus. 



Since no specimens of this species have been available for study 

 Twitchell 's description is given here verbatim : 



This little eehinoid, one of the smallest occurring in the Tertiary deposits 

 of the Pacific Coast, was first figured but not described by Ealph Arnold, of the 

 U. 8. Geological 8urvey, in whose honor the species is named. The test is very 

 small, being less than half an inch in diameter. In marginal outline it is sub- 

 oval, with a slight tendency toward the subpentagonal, the anterior end being 

 slightly angulated, the posterior broailer and roundeil. The whole form is much 

 depressed, slightly convex above, slightly concave below; margin of moderate 

 thickness. Ajjex central. The ambulacral petals are long, reaching the margin; 

 poriferous zones narrow, diverging in straight lines from apex to margin; pores 

 of both rows round. The whole test is covered with small but rather conspicuous 

 tubercles. The apical system is central, with a conspicuous tumid niadreporite. 

 The other details could not be made out on the specimen. The peristome is 

 relatively large, central, subcircular to subpentagonal. The ambulacral grooves 

 appear as rather well defined, simple, straight lines for a short distance out 

 from the peristome, lieyond which they become obscure. The periproet is very 

 small, circular, inframarginal, almost marginal. 



Dimensions. — Length 10.5 millimeters; width 9.5 millimeters. 



Kclutvd forms. — iS'. arnoldi resembles <S. {?) coalingaensis and Scutella (?) 

 merriami, but differs from both in having petals extending to the margin, with 

 straight, diverging jioriferous zones. 



Geologic horizon.- — Etchegoin formation. Pliocene. 

 Locality. — Four miles southeast of northwest end of Kettleman 

 Hills, Coalinga district, California. 



