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OOLITIC ECHINODERMATA. 
longer than the diameter of the shell ; surface of the solid forms covered with very fine, 
minute longitudinal striae ; and that of the tubular with obhque annulations of fringe-like 
scales. Jaws present. Range of family from the Trias to the existing period. Pamily 
represented in the British Liassic and Oolitic strata by four genera : Pseudodiadema, 
Hemipedina, Heterocidaris, and Pedina, in thirty-four species. 
Genus 5. — Pseudodiadema, Desor. (Vol. T, p. 108). 
Test moderately thick, not large. Ambulacral areas one third to one half the width of 
the interambulacral. Ambulacral areas furnished with two rows of primary tubercles. 
Interambulacral areas sometimes provided with two rows of primary tubercles, sometimes 
showing four or six rows of equal-sized tubercles at the equator. Pores of poriferous 
zones unigeminal throughout, or bigeminal in the upper part of the zones. Apical disc 
small. Oral opening large. Peristome deeply notched. Spines solid, cylindrical or 
needle-shaped, short, and covered longitudinally with very minute microscopic lines. 
Range of genus from the Lias to the Cretaceous beds. Thirteen British Liassic and 
Oolitic species (Vol. I, pp. 110—132, 456). 
Genus 6. — Hemipedina, Wrigld (Vol. I, p. 143). 
Test thin, much depressed on the upper surface, and flat or slightly concave on the 
lower surface. Ambulacral areas narrow. Interambulacral areas usually double the 
width of the ambulacral, with two to eight rows of perforated tubercles at the equator, 
having uncrenulated bosses ; upper part of miliary zone wide and covered with fine granu- 
lations. Pores of poriferous zones unigeminal. Spines slender and needle-shaped. Range 
of genus from the Lias to the Oolites. Eighteen British Liassic and Oolitic species 
(Vol. I, pp. 144—167, 457). 
Genus 7. — Heterocidaris, Cotteau (Vol. I, p. 455). 
Test thin, large, circular, depressed on the upper surface, almost flat on the lower 
surface. Ambulacral areas very narrow, furnished with two rows of small distinct per- 
forated tubercles, uniform in size, and raised on small bosses placed in regular rows. 
Interambulacral areas very wide, provided with from six to eight rows of large, nearly 
equal-sized perforated tubercles, raised on prominent bosses with crenulated summits. 
Miliary zone finely granulated. Poriferous zones narrow ; pores small, non-conjugate, 
having a slight disposition to a trigeminal arrangement near the oral opening. Oral 
opening large, pentagonal, and lobed. Anal opening circular. Spines long and cylindrical, 
