178 



OOLITIC ECHINODEEMATA. 



ovarian and five ocular plates, the right lateral plate large, and supporting a prominent 

 spongy madreporiform body. Spines small, short, and covered with microscopic lines. 

 The Echinoconidse range from the Lias to the existing period. Four British Oolitic 

 genera (Holectypus, Pygaster, Hyboclypus, and Galeropygus), with thirteen 

 British Oolitic species. 



Genus 14. — Holectypus, Desor (Vol. I, p. 259). 



Test thin, circumference circular, more or less hemispherical, depressed, always tumid 

 at the sides, flat or concave at the base. Ambulacral areas narrow and lanceolate, 

 bearing six to eight rows of small tubercles from base to apex. Interambulacral wide, 

 supporting numerous small perforated tubercles, arranged in vertical and concentric 

 rows. Poriferous zones narrow; pores unigeminal throughout. Oral opening very large, 

 circular, in centre of base. Peristome divided into ten equal lobes. Jaws present. Anal 

 opening very large, inferior, infra-marginal, rarely marginal, sometimes occupying the entire 

 space between the oral opening and the border. Apical disc nearly central and^ vertical. 

 The genus is found in the Oolites and Lower Cretaceous beds. Three British Oolitic 

 species (Vol. I, pp. 260—268). 



Genus 15. — Pygaster, Agassiz (Vol. I, p. 273). 



Test thick, subpentagonal, more or less elevated, convex on upper surface, concave on 

 under. Ambulcaral areas narrow, bearing four to six rows of small tubercles, the 

 marginal rows extending from base to apex. Interambulacral wide, bearing small, 

 nearly equal-sized, perforated, and uncrenulated tubercles, arranged in vertical and hori- 

 zontal rows, of which the two are median, and continuous throughout. Poriferous zones 

 narrow ; pores unigeminal throughout. Oral opening circular. Peristome deeply notched 

 and marked by ten equal lobes. Jaws present. Anal opening very large, oblong, superior, 

 and almost universally continuous with apical disc. Apical opening central, large. 

 Spines small, short, covered with minute longitudinal lines. Range of genus from 

 the Lias to the existing period. Pive British Oolitic species (Vol. I, pp. 275 — 282, 

 463). 



Genus 16.^ — Hyboclypus, Agassiz (Vol. I, p. 291). 



Test thin, subcircular or subovate, upper surface unequally elevated, the anterior 

 half usually higher than the posterior half. Under surface concave or undulating. 

 Ambulacral areas narrow, flexuous, and disjointed at summit by the length of the apical 

 disc. Three terminate at the oral extremity of the apical disc, two at the opposite end. 



