182 MESOZOIC AND CENOZOIC ECHINODERMATA OF THE UNITED STATES. 



to the apical system. A third row lies between the other two, and in the larger forms there are 

 additional tubercles that are best developed near the ambitus and do not reach either peristome 

 or apical system. The poriferous zones are narrow, and the pore pairs are found in triplets 

 arranged obliquely. 



The interambulacra are covered with tubercles similar to the ambulacra. Six or more rows 

 of primary tubercles occur, four being very regular and two reaching all the way from the peri- 

 stome to the anal system. The remaining two rows and in some cases additional rows or extra 

 subequal tubercles are chiefly developed at the ambitus. ' The entire surface of both inter- 

 ambulacra and ambulacra are thickly studded with so many tubercles of about the same size as 

 to give it a very uniform appearance. 



The apical system is small. The peristome is rather wide with moderately distinct branchial 

 incisions. 



Localities. — James Kiver and York River (type and figured specimen), Va. 



Geologic Tiorizon. — Yorktown formation, upper Miocene. 



Collections.— Johns Hopkins University (T 1001) ; Academy of Natural Sciences of Phila- 

 delphia. 



Psammechinus exoletus McCrady. 



Psammechinus exoletus McCrady, 1857, in Tuomey and Holmes, Pliocene fossils of South Carolina, p. 4, PI. II, fig. 6. 



Psammechinus exoletus Meek, 1864, Check list, Miocene, p. 2. 



Psammechinus exoletus Stefanini, 1911, Soc. geol. italiana Boll., vol. 30, p. 707. 



Description. — The fragment briefly described and figured by McCrady is the only material 

 so far as known that has been found representing this species. The description and figure are 

 insufficient to separate with certainty this form from other species. 



Locality. — Goose Creek, S. C. (Smith.) 



Geologic Tiorizon. — Duplin marl, upper Miocene. 



Collection. — Unknown. 



Subclass IRREGULARIA. 



Order GNATHOSTOMATA. 



Suborder CLYPEASTRINA. 



Family FIBULARIIDiE. 



Genus SISMONDIA Desor. 



Sismondia (?) arnoldi Twitchell, n. sp. 



Plate LXXXV, figures la-b. 



Astrodapsis sp. indet. Arnold, 1909, TJ. S. Geol. Survey Bull. 396, p. 30, PI. XXVIII, fig. 5, 5a. 

 Astrodapsis sp. indet. Stefanini, 1911, Soc. geol. italiana Boll., vol. 30, p. 703. 



Description and determinative characters. — This little echinoid, one of the smallest occurring 

 in the Tertiary deposits of the Pacific coast, was first figured but not described by Ralph Arnold, 

 of the U. S. Geological Survey, 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 suboval, with a slight tend- 

 ency toward the subpentagonal, the anterior end being slightly angulated, the posterior broader 

 and rounded. The whole form is much depressed, slightly convex above, slightly concave below; 

 margin of moderate thickness. Apex 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 madreporite. 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, beyond which they become obscure. The periproct is very small, circu- 

 lar, inframarginal, almost marginal. 



Dimensions. — Length 10.5 millimeters; width 9.5 millimeters. 



