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



STOMECHIXUS Besor. 



Stomechinus Hyatti Clark. 



Plate XXIII, Figs. la-e. 



Determinative characters. — Test high, eircnrofereiice circular. Ambu- 

 lacra with four rows of small primary tubercles ; poriferous zones wide, 

 trigeminal. Intorambulacra Avith eight or ten rows of tubercles that 

 diminish in number toward apical disk and mouth o]>ening, with a de- 

 pression along the line of the central suture. 



Dimensions. — Height, 1 inch; transverse diameter, 1 inch. 



Description. — The single specimen of this interesting Jurassic form has 

 only the cast of a portion of the exterior preserved. The test is high, 

 and the circumference circular or slightly subpentagonal. The sides 

 rise abruptly from the margin (PL xxiii. Fig. la). 



The ambulacra! areas are about one-half tlie width of the interam- 

 bulacral, and preserve a nearly uniform width throughout. There are 

 four rows of tubercles that increase slightly in size from above down- 

 ward. A depression appears along the line of the central suture. 

 The poriferous zones are wide; the iiore pairs are placed in oblique 

 ranks of three, and each rank is separated from that next to it by two 

 small tubercles (PI. xxiii, Fig. Ih). 



The interambulacral area is about twice the width of the ambulacral, 

 and the plates of the former are but slightly higher than those of the 

 latter (PI. xxiii. Fig. Ic). Each plate is covered at the widest part of 

 the area with an irregulai' row of four or five tubercles that are of about 

 jpqual size. The tubercdes of the interambulacral areas are of about the 

 same size as those of the ambulacral. The tubercles are uncrenulated 

 and imperforate. A wide depression extends along the medial line of 

 each interambulacral area according with the position of the central 

 suture. 



The regions of the apical disk and mouth opening are largely want- 

 ing, but a deep incision shown upon the margin of the latter indicates 

 that it is distinctly lobed throughout. 



Belated forms. — The American species shows some points of identity 

 with ^StomecMnus nuclus Wright of England, though the latter is less 

 elevated, has a sharper marginal angle, and a less regular arrangement 

 of the tubercles. 



Locality and geological liorizon. — This species is trom the Jurassic 

 strata of Taylorsville, California. 



Collection. — U. S. National Museum. 



