44 MESOZOIC ECHINODERMATA OF THE. UNITED STATES. [bull. 97. 



Lovalifi/ and f/eologlc((l horizon. — This species, like ^'. fninidula, is 

 only found in the yellow limestone of the middle marl bed (upper Cre- 

 taceous) of Timber creek, New Jersey. 



Collections. — Phihidelphia Academy of Natural Sciences ; American 

 Museum of Natural History, New York. 



DIABEMATID^. 



Test more or less spheroidal. Ambulacral areas broad, at times ap- 

 proaching the interambulacral in width. The ambulacral i)lates are 

 generally compound; the pores unigeminal, increasing oftentimes in 

 the vicinity of the peristome. The mouth opening is large, with deep 

 incisions, and provided near the margins witli well-developed auriculae. 

 The spines are frequently large and variable in shape. 



HEMICIDARIS Agassiz. 



Hemicidaris intumescens Clark. 



Plate XII, Figs. ]a-r. 



Determinative characters. — Test spheroidal, upper surface inflated, 

 lower surface flat. Ambulacral areas moderately broad, increasing in 

 'width from the apical disk to the mouth edges. The tubercles at the 

 peristome are nearly equal in size to those of the interambulacral areas, 

 but become greatly reduced in passing from the ambitus to the apical 

 disk. Interambulacral areas rather wide, with two rows of large tuber- 

 cles. Miliary space Avide, covered with numerous granules. IMouth 

 opening moderately wide, with deep incisions. 



Dimensions. — Height, ^ inch; transverse diameter, f inch. 



Description. — The very perfectly preserved molds of this species permit 

 the taking in gutta-percha of all the details of form and structure. As 

 the only representative of this genus in American deposits it possesses 

 considerable interest. The test is of moderate size, with a convex and 

 slightly inflated upper surface and nearly flat lower surface. The 

 slightly flexuous ambulacral areas broaden from the region of the api- 

 cal disk toward the ambitus and slightly contract again before the 

 peristome is reached. The tubercles are prominent in the lower part 

 of the column, where several pairs attain a size nearly equal to those 

 of the interambulacral areas. Minute perforated granules succeed them 

 above and continue as a double row to the apical disk. The poriferous 

 zones are slightly flexuous, and the small jjores are unigeminally ar- 

 ranged, although somewhat increased in the vicinity of the mouth 

 edges. 



The interambulacral areas are from three to four times the width of 

 the ambulacral at the ambitus. The two rows of medium sized tuber- 

 cles occupy the center of the plates; the areola is circular and slightly 

 elevated; the boss deeply crenulated, and the mamelon distinctly per- 



