JURASSIC ECHINODEEMATA. 33 



Family PEDIXID/E. 



Genus STOMECHINUS Desor. 



Stomechinus hyatti Clark. 



Plate V, figures 2a-e. 



Stomechinus hyatti Clark, 1893, Johns Hopkins Univ. Circ, vol. 12, No. 103, p. 51. 

 Stomechinus hyatti Clark, 1893, U. S. Geol. Survey Bull. 97, p. 56, PI. XXIII, figs. la-e. 



Determinative characters. — Test high, circumference circular. Ambulacra with four rows 

 of small primary tubercles; poriferous zones wide, triserial. Interambulacra with eight or ten 

 rows of tubercles that diminish in number toward apical system and peristome, with a depression 

 along the line of the central suture. 



Dimensions. — Diameter 2.5 millimeters; height 25 millimeters. 



Description. — Only the cast of a portion of the exterior of a single specimen of this interest- 

 ing Jurassic form is preserved. The test is high, and the circumference circular or slightly sub- 

 pentagonal. The sides rise abruptly from the margin. 



The ambulacra are about one-half the width of the interambulacra and preserve a nearly 

 uniform width throughout. There are four rows of tubercles that increase gradually in size 

 from above downward. A depression appears along the line of the central suture. The porif- 

 erous zones are wide; the pore pairs are placed in oblique ranks of three, and each rank is 

 separated from that next to it by two small tubercles. 



The interambulacra are about twice the width of the ambulacra and the plates of the former 

 are but slightly higher than those of the latter. Each plate is covered at the widest part of the 

 area with an irregular row of four or five tubercles that are of about the same size as those of the 

 ambulacra. 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 peristome are largely wanting, but a deep incision shown 

 upon the margin of the latter indicates that it is distinctly lobed throughout. 



Related forms. — The American species shows some points of identity with Stomechinus nudus 

 Wright of England but is more elevated, has a less sharp marginal angle, and has a more regular 

 arrangement of the tubercles. 



Locality. — Taylorsville, Plumas County, Cal. 



Geologic horizon. — Mormon sandstone, Middle Jurassic. 



Collection. — U. S. National Museum (30185). 



Subclass IRREGULARIA. 



Order GNATHOSTOMATA. 



Suborder HOLECTYPINA. 



Family PYGASTERID^E. 



Genus HOLECTYPUS Desor. 



Holectypus pealei Clark, n. sp. 



Plate V, figures 3a-b. 



Determinative characters. — Test subcircular, subcorneal. Ambulacra rather narrow, nearly 

 straight; poriferous zones distinct. Interambulacra prominent with large plates. Apical system 

 imperfect. 



Dimensions. — Diameter 15 to 25 millimeters; height 10 to 15 millimeters. 



Description. — Several casts of a typical Holectypus from the Jurassic beds of Yellowstone 

 River were collected by Peale in Montana many years ago but were never described. . More 

 recently a still more perfect specimen was obtained by W. H. Weed from the Yellowstone 

 National Park, and this is made the type of the species. The specimens show the characteristic 

 features of both the ambulacra and interambulacra, but the adoral and aboral regions are poorly 

 preserved, as a result of which the characters of the apical s3 T stem, peristome and periproct are 



