180 MESOZOIC AND CENOZOIC ECHINODEKMATA OF THE UNITED STATES. 



Subclass REGULARIA ECTOBRANCHIATA. 



Order DIADEMOIDEA. 



Suborder ARBACINA. 



Family ARBACIID^E. 



Genus ECHINOCIDARIS Duncan. 



Echinocidaris sp. McCrady. 



Echino-cidaris sp. McCrady, 1855, in Tuomey and Holmes, Pliocene fossils of South Carolina, PL II, figs. 5, 5a. 

 Arbacia sp. Stefanini, 1911, Soc. geol. italiana Boll., vol. 30, p. 707. 



Description. — An illustration of this form "restored from several fragments" is given by 

 McCrady but no description is furnished. Its generic relations are very questionable because of 

 doubt as to the accuracy of the drawing. 



Locality. — South Carolina. 



Geologic horizon.— Miocene (?). 



Collection.— Unknown. 



Genus CCELOPLEtTBtTS Ag-assiz. 



Ccelopletjrus imphocertjs (Conrad). 



Plate LXXXIV, figures 4a-c. 



Echinus improcerus Conrad, 1843, Acad. Nat. Sci. Philadelphia Proc, vol. 3, p. 310. 

 Psammechinus improceras Stefanini, 1911, Soc. geol. italiana Boll., vol. 30, p. 705. 



Determinative characters. — Test large, circular, depressed, concave below. Ambulacra 

 narrow, each with two rows of primary tubercles extending from peristome over ambitus about 

 two-thirds of way from latter to apical system, secondary tubercles above; poriferous zones 

 narrow, slightly nexuous. Interambulacra wide, the two rows of primary tubercles on upper 

 surface small. Peristome large, with slight brachial incisions. 



Dimensions. — Diameter 35 millimeters; height 15 millimeters. 



Description. — This species was first described by Conrad in 1843. No illustration accom- 

 panies the description, but the species is without doubt the form found by the authors in the 

 Yorktown formation near the James River and figured and described in this monograph. It 

 corresponds in all particulars to the incomplete description which Conrad gives to it. 



The test is large, circular and depressed. The slope of the upper surface is very low toward 

 the apex. The lower surface is concave. 



The ambulacra are narrow, each bearing two rows of primary tubercles which are much 

 larger at the ambitus than on the lower or upper surface. They diminish rapidly in size on the 

 upper surface and about two-thirds of the way to the apical system become secondaries or 

 diminish altogether. One or more isolated primaries also occur at this point of disappearance 

 with a secondary between. The poriferous zones are narrow and somewhat sinuous, the p>ore 

 pairs uniserial. 



The interambulacra are broad with four rows of primary tubercles at the amibitus similar 

 to those of the ambulacra. They become smaller, reduced in number, and more or less irregularly 

 crowded together near the peristome. Above the ambitus the rows of primary tubercles are 

 soon reduced to two which with diminished size continue to the apical system. A large, miliary 

 median space is found in the upper portion of each interambulacrum. 



The apical system is large but much broken in the type specimen. The peristome is large 

 with small branchial incisions. The periproct is large and elongate. 



Localities. — James River, near Smithfield (type) ; J. T. Williams's marl pit, Smith Creek, 

 half a mile below Suffolk, Va. (figured specimen) . 



Geologic horizon. — Yorktown formation, upper Miocene. 



Collection. — U. S. National Museum (1664S7). 



