<iAiu<] PSEUDODIADEMA EMERSONI. . 45 



forated. There are about eight piiiiiary tiiberck's in each row. The 

 apical disk is rather hirge, although not suiHcicutly well preserved to 

 admit of a deteriniuatiou of the individual plates. 



The mouth opening is small and occupies scarcely one-half of the di- 

 ameter of the test. It is deeply notched, the lobes being nearly equal 

 in size. 



Related forms. — The present species of Hemicidaris is the only repre- 

 sentative of the genus known from American dei)()sits. A comparison 

 with European species fiiils to identify it with any described forms. 



LoeaUtii and (jeoUxjlcal horizon. — This species is from the Jurassic 

 strata of Taylorsville, California. 



Collection. — U. S. National Museum. 



PSEUDODIADEMA Desor. 



PSEUDODIADEMA Emersoni Clark. 



Plate XII, Figs 2,a-e. 



Determinative characters. — Test circular, depressed. Ambulacral 

 areas straight and prominent; poriferous zones narrow; pores unigemi 

 nal. luterambulacral areas not quite twice the width of the ambu- 

 lacral, with two rows of large tubercles, eight or nine in each row. 

 Mouth opening wide. 



Dimensions. — Height, ^ inch; transverse diameter, f inch (?). 



Description. — The beautifully preserved molds of this interesting 

 form were collected by Prof. Hyatt from the Jurassic beds of Taylors- 

 ville, California. The gutta percha casts from which the drawings 

 were made represent very perfectly all the details of structure. The 

 test is nearly circular, the prominent ambulacral areas giving it a 

 slightly subpentagonal outline. The ambulacral areas are rather more 

 than one-half the width of the interambulacral, 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 ai)ical disk the tubercles are 

 greatly reduced in size. The pores are unigeminal, there being three 

 or four opposite each ambulacral plate. 



The interambulacral areas 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 arc nearly contluent above 

 and below, broader than long, and rise into prominent bosses that are 

 not deeply crenulated. The mamelon is distinctly perforatcil. Tlie 

 miliary space is covered with granules of diftereut sizes. There are no 

 secondary tubercles developed near the month opening. The lower sur- 



