233 HOLECTYPUS 



The anal opening is large, inferior, infra-marginal, rarely marginal, sometimes 

 occupying the entire space between the mouth and the border. 



The apical disc is nearly central and vertical, composed of five ovarial and five 

 ocular plates ; the right antero-lateral ovarial is much the largest and extends into the 

 centre of the disc ; it supports a prominent, convex, madreporiform body. In all the 

 Oolitic species the anterior and posterior pairs of ovarials are perforated, and the single 

 plate imperforate; whilst in all the Cretaceous species the five ovarial plates are all 

 perforate, and the five ocular plates are small, triangular bodies, with marginal perfora- 

 tions (fig. 1 i). 



The internal moulds of Hohdypus want those depressions occasioned by ribs 

 projecting from the inner walls of the test which so well characterise the genus Discoidea. 



The spines are short, with a smooth head and milled ring, and they have the surface 

 sculptured with fine longitudinal lines. 



Iloledypus is distinguished from Echinocoiim by having a larger mouth and vent, 

 a concave base, and a less elevated dorsal surface; and from Discoidea in having 

 tumid sides, a larger mouth and vent, and the absence of ribs from the internal walls 

 of the test. 



The small crenulated tubercles and basal vent, with the absence of any aperture 

 in the upper surface of the inter-ambulacrum, distinguishes Iloledypus from P yg aster ; 

 and the want of a longitudinal valley in the inter-ambulacrum separates Iloledypus from 

 Hyboclypus and Galeropygus. 



The Genus Iloledypus is most abundant in the Oolitic rocks; the Cretaceous 

 rocks of France contain seven species : one is special to the Neocomian, one to the 

 Aptien, three to the Cenomanian,' and two to the Turonian stages. I now add a new 

 species from the Chloritic Marl of England, and the first of this genus from the chalk 

 found in the British Islands. 



The Genus Holedypus forms two natural groups, both organically and stratigraphically 

 distinct from each other. The apical disc in one group has only four of the ovarial 

 plates perforate ; in the second group all the five ovarials are so. The species with the 

 four perforate ovarials are all Jurassic, and those with the five perforate ovarials are 

 Cretaceous. 



