4G MELSOZOIC ECHINODERMATA OF THE UNITED STATES, [bull. 97. 



face is flat, the mouth opening large aud deeply notched j the ambu- 

 lacra! lobes larger than the interambulacral. The apical disk is not 

 preserved. 



Belated forms. — As the only Pseudodiadema in the Jurassic of America 

 it is unique, while its identity with any European form is very doubtful. 



It closely resembles P. Baihj/i, from England, in many particulars, 

 but it is a less depressed form, with more rounded sides aud more 

 numerous granules in tlie miliary space. 



Locality and geological horizon. — This species is found in the Jurassic 

 beds of Taylorsville, California, and is named in honor of Prof. B. K. 

 P_]merson, of Amherst College, from whom the writer received his first 

 instructions in geology and paleontology. 



Collection. — U. S. National Museum. 



Pseudodiadema diatretum (Morton). 



Plate XIII, Figs. la-f. 



■ Cidaris diatretum Morton, 1833. Am. Jour. Sci., Ist ser., vol. 23, p. 294. 



. Cidarites diatretum Morton, 1834. Synop. Organ. Remains, Cretaceons, p. 75, PI. 10, 



Fig. 10. 

 'Cidaris diatretum Bronn, 1848, Index Pal., vol. 1, p. 298. 



Pseudodiadema diatretum Desor, 1858. Synopsis des fichinides fossiles, p. 73. 



Cidaris diatretum Gabb, 1859. Cat. Invert. Fossils, Cretaceous, p. 18. 



I'scudodiadema diatretum Cotteau, 1862-67. Paleont. fraufaise, vol. 7, p. 519. 



I'seudodiadema diatretum Meek, 1864. Smith. Misc. Coll., vol. 7 (177), p. 2. 



rseudodiadema diatretum Conrad, 1868. Geol. ofN. J., Appendix, p. 722. 



Cidaris clavigera Creduer, 1870. Zeitsclir. d. deutscli. geol. Gesells., vol. 22, p. 21.8. 



Cidaris sceptrifera Credner, 1870. Zeitsclir. d. deutscli. geol. Gesells., vol. 22, p. 218. 



I'seudodiadema diatretum Clark, 1891. Johns Hopkins University Circulars, No. 87, 

 p. 75. 



Determinative characters. — Test circular, convex above, concave be- 

 low, sides inflated. Ambulacra wide, sinuous, with two rows of tuber- 

 cles, that are large at the ambitus and decrease toward the poles; 

 poriferous zones narrow, siuuous. 'Interambulacra with two rows of 

 jirimary tubercles and several irregular rows of secondary tubercles. 

 Mouth opening narrow, about one-third the diameter of the test. 



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



Description. — The specimens of this species vary greatly in size, the 

 full-grown forms being moderately large. The test is circular, depressed, 

 slightly convex above, concave below. The sides are inflated (PI. xiii, 

 Fig. Ih). 



The ambulacral areas are wide, slightly raised, and furnished with 

 two rows of tubercles, thirteen to fourteen in each row. They are 

 large and prominent at the ambitus, but decrease gradually toward 

 the poles. Tlie broad areolas, that are sharply depressed in their mar- 

 ginal portions, are striated by faint radial ridges on the outer sides. 

 At the upper and inner angle of each plate is a small secondary tuber- 

 cle, while irregularly scattered over the other portions of the plates are 



