32 MESOZOIC AND CENOZOIC ECHINODEEMATA OF THE UNITED STATES. 



poriferous zones are slightly flexuous, and the small pores are uniserially arranged, although 

 somewhat increased in the vicinity of the peristome. 



The interambulacra are three to four times the width of the ambulacra at the ambitus. The 

 two rows of medium-sized tubercles occupy the center of the plates; the areola is circular and 

 slightly elevated; the boss deeply crenulated, and the mamelon distinctly perforated. There 

 are about eight primary tubercles in each row. The apical system is rather large, although not 

 sufficiently well preserved to admit of a determination of the individual plates. 



The peristome is small and occupies scarcely one-half of the diameter of the test. It is 

 deeply notched, the lobes being nearly equal in size. 



Related forms. — The present species of Hemicidaris is the only representative of the genus 

 known from American deposits. A comparison with European species fails to identify it with 

 any described form. 



Locality. — Taylorsville, Plumas County, Cal. 



Geologic horizon. — Mormon sandstone, Middle Jurassic. 



Collection.— U. S. National Museum (30187 and 30188). 



Suborder DIADEMINA. 

 Family DIADEMATIDtf]. 



Genus PSETJDODIADEMA Desor. 

 PSETTDODIADEMA EMERSONI Clark. 

 Plate V, figures la-e. 

 Pseudodiademaemersoni Clark, 1893, U. S. Geol. Survey Bull. 97, pp. 45, 46, PL XII, figs. 2a-e. 



Determinative characters. — Test circular, depressed. Ambulacra straight and prominent; 

 poriferous zones narrow; pores uniserial. Interambulacra not quite twice the width of the 

 ambulacra with two rows of large tubercles, eight or nine in each row. Peristome wide. 



Dimensions. — Diameter 15 millimeters; height 7 millimeters. 



Description. — The beautifully preserved molds of this interesting form were collected by 

 Hyatt from the Jurassic beds of Taylorsville, Cal. The gutta-percha casts from which the draw- 

 ings were made represent very perfectly all the details of structure. The test is nearly circular, 

 the prominent ambulacra are rather more than one-half the width of the interambulacra, and 

 support two rows of primary tubercles somewhat smaller than the interambulacral tubercles. 

 There are nine or ten in each row. Between the tubercles are numerous granules that are 

 arranged in an undulating line between the rows and along the margin. They nearly disappear 

 between the tubercles of the same series. Toward the apical system the tubercles are greatly 

 reduced in size. The pores are uniserial, there being three or four opposite each ambulacral 

 plate. 



The interambulacra bear two rows of primary tubercles, eight or nine in each, that are only 

 slightly larger than those of the ambulacral areas. They increase from the peristome to the 

 ambitus, after which they rapidly decrease in size. The areolas are nearly confluent above and 

 below, broader than long, and rise into prominent bosses that are not deeply crenulated. The 

 mamelon is distinctly perforated. The miliary space is covered with granules of different sizes. 

 There are no secondary tubercles developed near the peristome. The lower surface is flat, the 

 peristome large and deeply notched; the ambulacral lobes larger than the inter ambulacral. The 

 apical system is not preserved. This species is named in honor of B. K. Emerson, of Amherst 

 College, from whom the writer received his first instruction in geology and paleontology. 



Related forms. — The species is unique, being the only Pseudodiadema in the Jurassic of 

 America and being only very doubtfulty identical with any European form. It resembles P. 

 baileyi from England in many particulars but is less depressed, has more rounded sides, and more 

 numerous granules in the miliary space. 



Locality. — Taylorsville, Plumas County, Cal. 



Geologic horizon. — Mormon sandstone, Middle Jurassic. 

 Collection. — U. S. National Museum (30186). 



